The most important nutrients for the thyroid: iron, selenium, zinc, vitamin D
Your thyroid builds its hormone in a chain of work steps. Each step needs its own helper. The one most often overlooked is iron, and a ferritin that counts as normal in the lab can still be too low for this chain.
All guides in the thyroid cluster
A ferritin of 25 is above the classic threshold of 15, the point below which iron deficiency was long said to begin. The report then says: unremarkable. Unremarkable does not mean sufficient. Least of all for a thyroid that carries iron at the center of its most important enzyme.
You wake up and you are already tired. Not a little. As if you had worked through the night.
Your hands are cold, even when it is warm outside. More hair stays in your hairbrush than it used to. In the afternoon your head feels as if it were wrapped in cotton wool.
You think of your thyroid. Maybe it has already been checked. Result: within the normal range. Your blood count too. And yet you sense that something is not right.
I want to tell you something before we go into the biochemistry. You are not imagining this feeling. It deserves a second question.
“Everyone says my thyroid is fine.”
A woman in her early forties, under heavy demands at work and at home. She did not come because of her thyroid, but because of a fatigue that had not gone away for months.
The thyroid values measured so far were within the reference range. So was her hemoglobin. Her ferritin was 25 µg/L, and the report said: unremarkable. Until then, nobody had talked to her about her heavy periods.
We did not start by adjusting her thyroid. We asked why her iron store was so empty. We had the cause investigated medically and then refilled the stores in a targeted way, until they were above the value I aim for when someone has symptoms.
A few months later she told me that she finds her way into the day again in the morning and feels the cold less. I cannot claim causality, but I document the temporal association.
The lesson: A value within the reference range does not answer the question of whether it is enough for you.
This description is a pattern drawn from many conversations. It contains no data from any single person and is not a treatment protocol.
You will find the overview of each individual building block in Selenium, zinc, iron and vitamin D: the building blocks of the thyroid. How to put selenium in Hashimoto's into context is covered in detail in Selenium and the thyroid. This article is about something else: the chain as a whole, the order, and the building block that is most often overlooked.
To make its hormone, the thyroid needs a chain of nutrients: iodine as the raw material, iron in the enzyme thyroid peroxidase, selenium as protection and for converting T4 into T3, zinc at several switch points, and vitamin D in the immune system. Iron is the one most often overlooked. In meta-analyses, iron deficiency goes hand in hand with lower thyroid hormones and more frequent thyroid antibodies. Classic cutoffs often detect an empty store late. When someone has symptoms, I aim for a ferritin above 100 µg/L, always read together with transferrin saturation and inflammation markers. With selenium, zinc and iodine, on the other hand, more is not automatically better.
What to expect in this article
- The hormone chain: five stations, five helpers
- Iron at the center of the workbench
- What studies show about iron deficiency and the thyroid
- Why a “normal” ferritin can be too low
- My ferritin target above 100 and the reasoning behind it
- The vicious circle of hypothyroidism and iron deficiency
- Selenium, zinc, vitamin D and iodine: what the evidence supports
- Which values make the chain visible
- Three levers for your next doctor's appointment
Which nutrients does your thyroid need? The hormone chain in five stations
Key point: Thyroid hormone is not made from iodine alone, but in a chain of enzymes that need iron, selenium and zinc. In a study of children with goiter, the goiter persisted despite iodine in 64 percent of the children with iron deficiency anemia, but in only 12 percent of the children without anemia.
Maybe you once learned: thyroid equals iodine. That is not wrong. It is just one single link in a longer chain.
Imagine a small workshop. Raw materials are delivered. At a workbench, someone works with a welding torch. Next to it hangs a fire extinguisher. At the end, someone cuts the finished piece to size, and a courier takes it to where it is needed.
That is exactly how your thyroid works. And at almost every station there is a different nutrient.
Five stations, five helpers
Iodine and tyrosine. The thyroid takes up iodide from the blood. Tyrosine, a building block from the protein in your food, forms the framework of the hormone.
Thyroid peroxidase, TPO for short. It attaches iodine to the storage protein thyroglobulin. It is a heme enzyme and only becomes active once it has bound its iron-containing heme. Its tool is hydrogen peroxide, an aggressive molecule.
The fire extinguisher. Selenium-containing glutathione peroxidases intercept excess hydrogen peroxide so that the tool does not damage the gland's own cells.
The scissors. Deiodinases cut one iodine atom off the storage hormone T4, and this produces active T3. Deiodinases are selenoproteins. Reviews describe that zinc and iron can also influence their activity.
Key, lock and security service. T3 docks onto its receptor in the cell nucleus, which attaches to the genetic material with so-called zinc fingers. Zinc is also involved in making the control hormones TRH and TSH. Vitamin D plays a role mainly in the immune system, which in Hashimoto's attacks the body's own thyroid.
Summarized from Köhrle 2023, Zimmermann and Köhrle 2002, Hu and Rayman 2017, Szklarz and colleagues 2022, and Severo and colleagues 2019.
Shiqian Hu and Margaret Rayman from the University of Surrey compiled in the journal Thyroid in 2017 which nutrients may influence the risk of Hashimoto's.
They describe TPO as a heme enzyme that only becomes active at the surface of the thyroid cell after binding heme. According to their review, people with Hashimoto's often have iron deficiency, partly because autoimmune gastritis disrupts iron absorption. Their conclusion: doctors should check iron status, especially in menstruating women.
For you that means: with the thyroid, iron is not a side issue. It sits right in the middle of the workbench.
Hu S, Rayman MP. Thyroid. 2017;27(5):597-610. PMID: 28290237 · DOI: 10.1089/thy.2016.0635 [Review]The barrel with the shortest stave
There is an old image from plant science. A barrel made of wooden staves can only be filled up to the lowest stave. Everything above it runs out, no matter how tall the other staves are.
Your hormone chain works in a similar way. If iron is missing, a lot of iodine can achieve little. That sounds theoretical. A research team from ETH Zurich observed it in practice.
Researchers from ETH Zurich led by Michael Zimmermann and Sonja Hess gave iodine to children with goiter in an iodine-deficient region of Côte d'Ivoire and compared children with and without iron deficiency anemia. In a second, double-blind study, 166 children with goiter and iron deficiency received either iron or a placebo in addition to iodized salt.
After 30 weeks in the first study, 12 percent of the children without anemia still had a goiter, but 64 percent of the children with anemia. In the second study, the thyroid shrank by 22.8 percent on iron and by 12.7 percent on placebo, and a goiter remained in 43 versus 62 percent.
For you that means: the iodine arrived, but without iron less came of it. Children in an iodine-deficient region cannot be compared one to one with adults in Germany, but the principle of the chain shows up clearly here.
Zimmermann M et al. Am J Clin Nutr. 2000;71(1):88-93. DOI: 10.1093/ajcn/71.1.88 [Real-world intervention study, n=109] · Hess SY et al. Am J Clin Nutr. 2002;75(4):743-8. DOI: 10.1093/ajcn/75.4.743 [RCT, n=166]The question is not: which nutrient is the most important for the thyroid? It is: which link is the shortest for you right now?
If you only keep building on the tallest stave, the barrel will not fill any higher.
What does that mean in practice? If you feel as though you have an underactive thyroid, it is worth looking at the whole chain, not just at TSH and iodine. Why TSH alone is not enough for this is explained in TSH alone is not enough. You will find which values make the chain visible further down in this article.
And now you know why a thyroid with enough iodine can still be slowed down.
Can iron deficiency slow down the thyroid? What studies show
Key point: Iron deficiency can slow hormone production. In an animal model, TPO activity fell by up to 56 percent, and in humans, empty iron stores are associated in meta-analyses with lower free thyroid hormones and more frequent antibodies.
Maybe you are the person who never takes off their jacket at the office. Who brings a blanket to the cinema. Who hears from others: you get cold so easily.
That can have many reasons. Two of them are more closely linked than most people think: the thyroid and iron.
Sonja Hess and colleagues from ETH Zurich fed young rats diets with different degrees of iron deficiency for four weeks. Because iron deficiency reduces appetite, comparison animals received exactly the same amount of food, only with sufficient iron.
The more pronounced the iron deficiency, the more TPO activity fell, by 33, 45 and 56 percent. T3 and T4 were lower than in well-supplied animals, and the iron deficiency had an effect of its own beyond the reduced food intake.
For you that means: in an animal model, iron deficiency directly slows the hormone workbench. In humans this is biologically plausible, but it has not been shown this clearly.
Hess SY et al. J Nutr. 2002;132(7):1951-5. PMID: 12097675 · DOI: 10.1093/jn/132.7.1951 [In vivo, rat, n=84]J. L. Beard and colleagues from Pennsylvania State University placed women with iron deficiency anemia, women with empty stores but no anemia, and healthy women in a water bath at 28 degrees Celsius.
The women with anemia cooled down more, used less oxygen and had lower T4 and T3 values. After iron was given, they maintained their temperature better, and their thyroid hormones partly rose again. The women with empty stores but no anemia, by contrast, responded similarly to the control group in this small study.
For you that means: feeling cold can be a metabolic signal in which iron and the thyroid are both involved. But to be honest: the clearest effects here only appeared once anemia was present.
Beard JL, Borel MJ, Derr J. Am J Clin Nutr. 1990;52(5):813-9. PMID: 2239756 · DOI: 10.1093/ajcn/52.5.813 [Pathophysiology, human study, n=30]What the large analyses in humans show
Animal studies and small experiments are a start. What matters is what shows up in larger groups of people. Here are the numbers I consider the most important.
Vincenzo Garofalo and colleagues from the University of Catania pooled ten cross-sectional studies in 2023. A Beijing research group led by Jingyi Luo analyzed studies of women of reproductive age and pregnant women in 2021.
With iron deficiency, free T4 was on average 1.18 pmol/L lower and free T3 0.22 pmol/L lower, and thyroid antibodies were more frequent. Women of reproductive age with iron deficiency had 1.89 times the odds of TPO antibodies. Pregnant women with iron deficiency more often had subclinical hypothyroidism, with an odds ratio of 1.37, and more often overt hypothyroidism, with 1.60.
For you that means: iron deficiency and a weaker thyroid occur together strikingly often. But cross-sectional studies show associations, not causes, and the individual studies differed greatly.
Garofalo V et al. Nutrients. 2023;15(22):4790. DOI: 10.3390/nu15224790 [Meta-analysis] · Luo J et al. Front Endocrinol. 2021;12:629831. DOI: 10.3389/fendo.2021.629831 [Meta-analysis]Pregnancy has been studied particularly well, because the need for iron and the work of the thyroid rise at the same time. In a Swiss sample of 365 pregnant women, almost 40 percent had empty iron stores in the last trimester. Of the twelve women with a TSH above 4.0 mU/L, ten had empty stores, and the risk of a low T4 was 7.8 times higher.
In Brussels, Flora Veltri and colleagues examined 1,900 pregnant women in the first trimester. In 35 percent, ferritin was below 15 µg/L. Thyroid antibodies were found in 10 percent of them, and in 6 percent of the women without iron deficiency. A Belgian population study with 1,241 pregnant women concluded in 2021 that, with mild iodine deficiency, iron status rather than iodine was associated with free T4.
There are also data that do not fit the picture. A 2025 meta-analysis of 47 studies with 53,152 pregnant women found no clear association between ferritin and thyroid hormones, but did find one for hemoglobin. So the question of which iron value best reflects the thyroid is still open. In any case, a pregnancy belongs in the care of your gynecologist.
Hormonal system
According to reviews, iron deficiency can slow not only TPO, but also a deiodinase and the binding of T3 to its receptor in the cell nucleus. The chain then becomes slower at several points at once.
Metabolism
Iron is found in hemoglobin and in enzymes of the mitochondria, the power plants of your cells. If it is lacking, less heat and energy may be produced. That fits with the cold hands in Beard's study.
Nervous system
The brain needs iron for its messenger substances. Iron deficiency in the brain is considered one mechanism behind restless legs in the evening. So fatigue and that cotton-wool feeling in your head can come from two directions.
Immune system
Iron deficiency can throw the immune system off balance and is associated with more frequent thyroid antibodies. Conversely, inflammation holds iron in the stores through the hormone hepcidin.
Michał Szklarz and colleagues from the University of Olsztyn in Poland summarized in 2022 what role iron may play in autoimmune thyroid diseases. The title of their paper already says it: iron is not a passive bystander.
They describe that iron deficiency can lower the activity of TPO and of a deiodinase, inhibit the binding of T3 to its receptor and disturb the immune system. Iron deficiency, they write, could explain residual symptoms that remain in Hashimoto's even though hormone values are well adjusted.
For you that means: if you are exhausted despite good thyroid values, your iron store is an obvious lead to follow. That is a well-founded hypothesis, not proof for your individual case.
Szklarz M et al. Nutrients. 2022;14(21):4682. PMID: 36364944 · DOI: 10.3390/nu14214682 [Review]Feeling cold, hair loss and fatigue are not just thyroid signs. They are also iron signs.
Often it is both at once. Then it is worth pulling on both threads at the same time, instead of checking one and forgetting the other.
What you can take away from this: at your next blood test, do not just ask for a blood count. Ask for ferritin, transferrin saturation and CRP. And ask for the numbers, not just the word unremarkable. How iron, the thyroid and sleep can be connected is described in Iron deficiency, thyroid and sleep.
And now you know why cold hands may sometimes have more to do with your iron store than with the weather.
What ferritin level is good for the thyroid? Why “normal” can be too low
Key point: Where iron deficiency begins depends on how you measure it. In isotope studies of 1,058 women, the body already began absorbing more iron from food below about 50 µg/L, and an analysis of 29 guidelines recommends 100 µg/L for diagnosis in most situations.
Have you ever read your lab report and wondered who actually decides what is normal?
The reference range is a statistical window. It describes where most people in a comparison group fall. It does not tell you when your body starts to economize. And with ferritin, these two limits lie far apart.
Five numbers, five answers to the same question
The classic cutoff. This is how the WHO defines iron deficiency in healthy women. According to a 2025 analysis, this value is based on expert opinion.
Hemoglobin begins to fall. In data from 13,864 women from twelve countries, this point was at 24.8 µg/L. With this cutoff, 36 percent of the women had iron deficiency, with the WHO cutoff only 20 percent.
The body switches to saving mode. Below about 50 µg/L, it began absorbing more iron from food in isotope studies. In tired women without anemia and with a ferritin below 50, iron improved fatigue in a randomized trial.
The sleep medicine threshold. For restless legs, iron in tablet form is considered possibly effective up to a ferritin of 75 µg/L, according to an international guideline.
The threshold of many guidelines. An analysis of 29 guidelines recommends this value for diagnosis in most situations. In heart failure, chronic kidney disease and inflammatory bowel disease, a ferritin below 100 can count as iron deficiency.
My clinical target when there are symptoms. Always read together with transferrin saturation and CRP. A position from clinical practice, not a general guideline recommendation.
Sources: Addo and colleagues 2025, Galetti and colleagues 2021, Vaucher and colleagues 2012, Allen and colleagues 2018, Peyrin-Biroulet and colleagues 2015, Dignass and colleagues 2018.
Do you remember the woman from the story? Her ferritin of 25 was right at the point where, in the large multinational analysis, hemoglobin begins to fall. And it was well below the value at which the body, in the isotope studies, starts holding on to every iron atom from food.
Valeria Galetti and colleagues from ETH Zurich analyzed isotope studies of 1,058 healthy women between 18 and 50 years of age. Using labeled iron, they were able to measure how much iron the body actually absorbs from a meal.
Below a ferritin of 51.1 µg/L, absorption began to rise, and above it, absorption remained stable. That matched the hormone hepcidin, the doorkeeper of iron absorption, which had its threshold at about 3 nmol/L. The authors conclude that a ferritin below 50 µg/L indicates incipient iron deficiency in young women.
For you that means: your body itself signals when iron is getting scarce. In these measurements, it did so far above the classic cutoff of 15.
Galetti V et al. EClinicalMedicine. 2021;39:101052. PMID: 34401687 · DOI: 10.1016/j.eclinm.2021.101052 [Pathophysiology, pooled isotope studies, n=1,058]Laurent Peyrin-Biroulet and colleagues from Nancy and Paris compared 29 guidelines on iron deficiency from North America, Europe, Asia and Australia.
All guidelines used a ferritin value for diagnosis, and half also used transferrin saturation. Seven guidelines named a ferritin above 100 µg/L as a treatment target. The authors' conclusion: for diagnosis, a cutoff of 100 µg/L should be considered in most situations, with exceptions, for example in young healthy women with heavy menstrual bleeding.
For you that means: the question of the right value is still open among experts. And 100 is not an exotic number, but one that many medical societies use themselves.
Peyrin-Biroulet L, Williet N, Cacoub P. Am J Clin Nutr. 2015;102(6):1585-94. PMID: 26561626 · DOI: 10.3945/ajcn.114.103366 [Systematic review, 29 guidelines]Paul Vaucher and colleagues from the University of Geneva recruited 198 women through 44 general practices in France and assigned them by lot to two groups. All had unexplained fatigue, a hemoglobin above 12 g/dL and a ferritin below 50 µg/L.
After twelve weeks, fatigue had decreased by 47.7 percent on iron and by 28.8 percent on placebo. Quality of life, mood and anxiety, on the other hand, did not change measurably.
For you that means: without anemia, a store can be too empty for you to feel awake. The study involved menstruating women in general, not specifically people with a thyroid condition.
Vaucher P et al. CMAJ. 2012;184(11):1247-54. PMID: 22777991 · DOI: 10.1503/cmaj.110950 [RCT, n=198]Why I aim for a ferritin above 100
I want to be transparent here. A ferritin above 100 µg/L is my clinical target when someone has symptoms. It is not a general guideline recommendation for everyone. I base it on three lines of reasoning: physiology, the cutoffs used in other specialties, and an observation that concerns the thyroid directly.
Margaret Rayman from the University of Surrey summarized in 2019 what role iodine, iron and selenium may play in thyroid diseases.
She describes women who had persistent symptoms of hypothyroidism despite appropriately adjusted levothyroxine therapy. In two thirds of them, symptoms improved once their ferritin was back above 100 µg/L.
For you that means: this number is a clue, not proof. It comes from an observation cited in a review, not from a randomized trial. I am not yet aware of any randomized trial that tests exactly this target in women with thyroid conditions.
Rayman MP. Proc Nutr Soc. 2019;78(1):34-44. PMID: 30208979 · DOI: 10.1017/S0029665118001192 [Review]Cardiology shows how seriously other specialties take iron deficiency without anemia. There, the 100 µg/L cutoff was used in a large trial. And there, an open debate is now under way about whether ferritin alone is the right yardstick. Both belong to an honest answer.
A team led by Stefan Anker from Charité treated 459 people with heart failure and iron deficiency in the FAIR-HF trial, defined as a ferritin below 100 µg/L, or between 100 and 299 µg/L with a transferrin saturation below 20 percent. In 2024, a group of authors that included Anker himself proposed defining iron deficiency in heart failure in future primarily by a transferrin saturation below 20 percent.
On ferric carboxymaltose, a modern infusion preparation, 50 percent felt much or moderately better, compared with 28 percent on placebo, both with and without anemia. Side effects and serious side effects were similarly frequent in both groups. The later paper justifies its proposal by noting that inflammation can distort ferritin.
For you that means: iron deficiency without anemia can cause symptoms, as this trial in people with heart disease clearly shows. But ferritin alone is never the whole answer, which is why transferrin saturation always belongs alongside it.
Anker SD et al. N Engl J Med. 2009;361(25):2436-48. DOI: 10.1056/NEJMoa0908355 [RCT, n=459] · Packer M et al. Circulation. 2024;150(2):151-161. DOI: 10.1161/CIRCULATIONAHA.124.068883 [Review]Functional iron deficiency: when iron is there and still missing
There is a second level. Ferritin is not only a storage value. It is also an inflammation protein that rises during inflammation.
At the same time, during inflammation the hormone hepcidin closes the door of the intestinal wall and holds iron in the stores. Imagine a full warehouse whose gates are locked. On paper everything is there. And yet too little reaches the cells that need iron.
Axel Dignass and colleagues from Frankfurt described the limits of ferritin in inflammatory diseases in 2018. Sant-Rayn Pasricha and colleagues from Melbourne, Heidelberg and Nijmegen summarized the state of knowledge on iron deficiency in the Lancet in 2021.
Dignass and colleagues write that ferritin rises during inflammation and that the usual cutoff of 30 µg/L then no longer applies. They recommend measuring transferrin saturation as well. Pasricha and colleagues emphasize that clinical and functional impairments can occur even without anemia, and that inflammation holds iron back from the blood through hepcidin.
For you that means: a ferritin within the normal range does not rule out a functional deficiency. In Hashimoto's the immune system is active. How strongly that alone distorts ferritin has been little studied, which is why I always look at CRP and transferrin saturation too.
Dignass A, Farrag K, Stein J. Int J Chronic Dis. 2018;2018:9394060. DOI: 10.1155/2018/9394060 [Review] · Pasricha SR et al. Lancet. 2021;397(10270):233-248. DOI: 10.1016/S0140-6736(20)32594-0 [Review article]You will find the detailed reasoning behind my position in Ferritin above 100: a sensible target?. What a deficiency despite an unremarkable ferritin can mean is covered in Functional iron deficiency, and how inflammation locks the door in Iron, inflammation and the hepcidin blockade.
And if tablets are not enough?
In most cases, iron tablets are the first route. But the Lancet review also describes that high tablet doses can slow their own absorption through hepcidin, and so can inflammation. Then an infusion can be an option.
Iron infusions have a bad reputation with many people. This reputation stems largely from the era of older, high-molecular-weight preparations. The 2021 Lancet review describes that modern preparations allow the total dose to be given quickly and safely.
An infusion can be a good option when it is done correctly: with a clear indication, after ruling out iron overload such as in hemochromatosis, with a modern preparation and with good monitoring. Modern preparations also have side effects that you should know about and keep an eye on.
How you can recognize a carefully performed infusion is described in the Checklist for a well-done iron infusion. You will find the big overview of iron deficiency and infusions in Iron deficiency and iron infusions.
The reference range describes where many people fall. It does not describe where you feel well.
A value can be unremarkable and still too low for you. Both can be true at the same time.
And now you know why, when you are exhausted, I do not ask whether your ferritin is within the reference range, but whether it is enough for you.
Energy is not a luxury. Energy is freedom: the freedom to get up in the morning without already negotiating with your own body. When a single link in the chain is missing, it is not about a lab value. It is about your everyday life.
Can hypothyroidism itself lead to iron deficiency? The vicious circle
Key point: The thyroid and iron can slow each other down. According to a review, stomach disorders have been described in 10 to 40 percent of people with Hashimoto's, including autoimmune gastritis, which can disrupt iron absorption.
Maybe you have already taken iron, and your ferritin simply would not rise. That is frustrating. And it is often not down to you or to the tablet.
Sometimes there is a circle behind it. Iron deficiency slows the thyroid. And a sluggish thyroid can in turn worsen the iron deficiency.
How iron deficiency and hypothyroidism can feed each other
1 · Iron is missing
TPO works more slowly, and the conversion of T4 to T3 can also suffer.
on to 22 · The thyroid becomes sluggish
Less thyroid hormone can dampen blood formation in the bone marrow and the production of the hormone erythropoietin.
on to 33 · More loss, less absorption
Hypothyroidism is more often associated with heavy menstrual bleeding. Autoimmune gastritis lowers stomach acid, which iron needs in order to be absorbed.
on to 44 · The store empties further
The link where everything began becomes even shorter. And the circle starts again.
↻ back to 1Summarized from Szczepanek-Parulska and colleagues 2017, Krassas and colleagues 1999, Cellini and colleagues 2017, and Hu and Rayman 2017.
Ewelina Szczepanek-Parulska and colleagues compiled in a Polish journal of internal medicine in 2017 how the thyroid and blood formation are connected.
They describe that thyroid hormones stimulate the precursors of red blood cells directly and through erythropoietin, while iron deficiency impairs thyroid hormones. According to their review, anemia in hypothyroidism can arise from a dampened bone marrow, less erythropoietin, or a simultaneous deficiency of iron, vitamin B12 or folic acid. In autoimmune thyroid diseases, atrophic gastritis and celiac disease come in as additional causes.
For you that means: hypothyroidism and iron deficiency can be cause and consequence at the same time. If only one side is treated, the other is easily overlooked.
Szczepanek-Parulska E, Hernik A, Ruchała M. Pol Arch Intern Med. 2017;127(5):352-360. PMID: 28400547 · DOI: 10.20452/pamw.3985 [Review]A team led by G. E. Krassas from Thessaloniki asked 171 premenopausal women with hypothyroidism and 214 comparison women about their periods. A team led by M. Venerito from the University of Magdeburg examined the stomach in 34 people with autoimmune thyroid disease and 30 comparison subjects.
Irregular bleeding occurred in 23.4 percent of the women with hypothyroidism, but in only 8.4 percent of the comparison women, most often infrequent or very heavy bleeding. In Magdeburg, shrinkage of the acid-producing stomach lining was found in 23.5 percent of the people with autoimmune thyroid disease and in none of the comparison subjects.
For you that means: more blood loss and less stomach acid are two quiet ways in which an iron store can empty. Both can be checked by a doctor.
Krassas GE et al. Clin Endocrinol (Oxf). 1999;50(5):655-9. DOI: 10.1046/j.1365-2265.1999.00719.x [Real-world observation, n=385] · Venerito M et al. Aliment Pharmacol Ther. 2015;41(7):686-93. DOI: 10.1111/apt.13097 [Real-world observation, n=64]A review from Sapienza University describes this interplay as thyrogastric syndrome. According to it, around 40 percent of people with autoimmune gastritis also have Hashimoto's thyroiditis. Because the parietal cells of the stomach are lost, first the acid for iron absorption is lacking, and later also the factor needed for vitamin B12. And levothyroxine can then also be absorbed less well.
How acid blockers can additionally slow iron absorption is described in Acid blockers and iron absorption. And because celiac disease can also cost iron, the Lancet review recommends routinely considering a celiac test when iron deficiency is present.
Mohammadreza Ravanbod and colleagues from Bushehr in Iran treated 60 people with subclinical hypothyroidism and iron deficiency anemia in whom iron alone had not worked well enough. For three months, in a double-blind design, they received iron, levothyroxine or both.
Hemoglobin and ferritin rose more with the combination than with either one alone. TSH fell more in both groups receiving levothyroxine than with iron alone.
For you that means: sometimes it takes a look at both sides of the circle. What was measured here were blood values, not fatigue, and levothyroxine is a medication that belongs in medical hands.
Ravanbod M et al. Am J Med. 2013;126(5):420-4. PMID: 23582934 · DOI: 10.1016/j.amjmed.2012.12.009 [RCT, double-blind, n=60]A team from the University of Calgary asked 14 people with hypothyroidism whose treatment was well adjusted to take an iron tablet at the same time as their levothyroxine for twelve weeks.
Mean TSH rose from 1.6 to 5.4 mU/L, and nine of the 14 people had more signs of hypothyroidism. In the test tube, the iron bound to the hormone and formed a poorly soluble complex.
For you that means: iron and levothyroxine should be taken at separate times. Discuss the right interval with your doctor.
Campbell NR et al. Ann Intern Med. 1992;117(12):1010-3. PMID: 1443969 · DOI: 10.7326/0003-4819-117-12-1010 [Case series, uncontrolled study, n=14]If you take levothyroxine and want to supplement iron, agree on the interval and follow-up checks with your doctor beforehand. Any change can shift your TSH. Please never change your dose on your own.
An unexplained iron deficiency should be investigated by a doctor. In men and in postmenopausal women with iron deficiency anemia, the Lancet review recommends an endoscopy to rule out sources of bleeding in the gastrointestinal tract.
How to take levothyroxine sensibly in everyday life is described in Taking levothyroxine correctly. If you stay tired despite medication, you will find further leads in Tired despite levothyroxine.
If your ferritin does not rise despite tablets, that is not a failure on your part. It is a sign that a door is stuck somewhere in the circle: in the stomach, in the bleeding, in the inflammation or in how the tablets are taken.
And now you know why, with Hashimoto's and iron deficiency, it is worth searching the whole circle and not just the one value.
Does the thyroid need selenium? Fire extinguisher and scissors
Key point: Selenium protects the thyroid and activates its hormone, but more is not better. A meta-analysis of 35 studies found lower TPO antibodies on selenium, and the European Food Safety Authority sets the upper limit for adults at 255 micrograms per day.
Selenium is the best-known thyroid supplement. Maybe it is already in your cupboard. And maybe nobody has ever measured whether you are actually short of it.
In the chain, selenium has two jobs. It sits in the glutathione peroxidases, the fire extinguisher next to the workbench. And it sits in the deiodinases, the scissors that cut active T3 out of T4.
Valentina Huwiler and colleagues from Inselspital Bern and the University of Basel analyzed 35 randomized trials on selenium in Hashimoto's.
Selenium lowered TPO antibodies, pooled from 29 groups with 2,358 participants. In people without hormone therapy, TSH also fell slightly. Free T4 and free T3 did not change significantly, and side effects were about as frequent as in the control groups. The authors rated the certainty of the evidence as moderate.
For you that means: selenium can dampen immune activity against the thyroid. Whether that makes you feel better is a separate question, which the selenium article answers in detail.
Huwiler VV et al. Thyroid. 2024;34(3):295-313. PMID: 38243784 · DOI: 10.1089/thy.2023.0556 [Meta-analysis, 35 studies]The flip side: selenium has a narrow window. In 2023, the European Food Safety Authority set an upper limit of 255 micrograms per day from all sources, also for pregnant and breastfeeding women. The critical endpoint, of all things, is hair loss. According to the authority, the people who can exceed this limit are mainly those who regularly take high-dose supplements or regularly eat Brazil nuts.
Saverio Stranges and colleagues analyzed a large randomized trial from the eastern United States in which 1,202 people received 200 micrograms of selenium or a placebo for an average of 7.7 years.
On selenium, more people developed type 2 diabetes, with a hazard ratio of 1.55. This was most pronounced in those who already had high blood selenium at the start, with a hazard ratio of 2.70.
For you that means: diabetes was a secondary endpoint of the trial here, so it is a signal and not proof. But it is a good reason not to take selenium long term without a proven need.
Stranges S et al. Ann Intern Med. 2007;147(4):217-23. PMID: 17620655 · DOI: 10.7326/0003-4819-147-4-200708210-00175 [RCT, secondary analysis, n=1,202]And the order matters. Zimmermann and Köhrle describe that in regions with severe iodine and selenium deficiency, iodine supply must be adequate first, before selenium is added, so as not to worsen hypothyroidism. The chain does not forgive blind refilling in the wrong place.
Selenium is a fire extinguisher, not fuel. You hang a fire extinguisher where one is missing. Hanging a second one next to the first does not make the workshop any safer.
And now you know why, with selenium, measuring comes before taking.
What does zinc do for the thyroid? A helper at several switch points
Key point: Zinc is involved in the control hormones, the deiodinases and the T3 receptor, but the evidence on zinc supplements is thin. In a randomized trial with 68 women, free T3 rose in the zinc groups, but most differences between the groups were not significant.
Brittle nails, thinning hair, every infection knocks you out. With signs like these, many people think of zinc. And indeed, zinc appears several times in the thyroid chain.
A review from the Federal University of Piauí in Brazil describes three switch points. Zinc is involved in making the control hormones TRH and TSH. It influences the activity of the deiodinases. And it shapes transcription factors, meaning proteins that switch genes on and off. The T3 receptor in the cell nucleus also holds on to the genetic material with zinc fingers.
Salma Mahmoodianfard and colleagues from Tehran divided 68 overweight women with hypothyroidism into four groups: zinc with selenium, zinc alone, selenium alone, or a double placebo, each for twelve weeks.
In the zinc groups, free T3 rose, and with zinc alone the ratio of free T3 to free T4 rose as well. For most hormone values, however, there were no significant differences between the groups, and zinc and selenium levels in the blood did not change measurably.
For you that means: zinc could play a role in converting T4 to T3. This small study is not enough to support a recommendation without a proven deficiency.
Mahmoodianfard S et al. J Am Coll Nutr. 2015;34(5):391-9. PMID: 25758370 · DOI: 10.1080/07315724.2014.926161 [RCT, double-blind, n=68]Zinc has a flip side too. A 2026 systematic review compiled 37 cases in which too much zinc damaged the blood count. Daily amounts ranged from about 50 to over 1,500 milligrams, over weeks to years. Almost all those affected had anemia, and copper was low in all of them. After stopping zinc and with copper, the blood count usually recovered within weeks to months.
So a nutrient that many people take for their immune defenses can, of all things, cause anemia through copper deficiency. How zinc, copper and selenium stay in balance is described in Minerals in balance.
Zinc is a trace element. And traces are easy to exceed. Overloading one link in the chain can make another one shorter.
And now you know why zinc belongs in the chain, but not in every jar.
Does vitamin D lower Hashimoto's antibodies? The conductor in the immune system
Key point: Vitamin D plays a role mainly in the immune system, less in the hormone workshop itself. In a meta-analysis of 20 studies, people with autoimmune thyroid disease had almost three times the odds of vitamin D deficiency.
In winter everything turns a little grayer. You are more tired, your mood sinks, and someone says: just take vitamin D. With Hashimoto's you hear this especially often.
Through the KPNI lens, vitamin D belongs mainly to the immune system. It does not build hormone and does not cut off iodine. But it can help determine how strongly the immune system works against your own thyroid.
Jiying Wang and colleagues from Luzhou pooled twenty case-control studies in 2015. Hui Jiang and colleagues from Zhenjiang analyzed six studies in 2022 with 258 people with Hashimoto's who had received vitamin D or a control treatment.
People with autoimmune thyroid disease had lower vitamin D levels and 2.99 times the odds of a deficiency. On vitamin D, TPO antibodies fell, while TSH, free T3 and free T4 did not differ significantly from the control group.
For you that means: vitamin D could dampen immune activity, but hormone values changed little in these studies. And Hu and Rayman consider it more likely that a low vitamin D is a consequence rather than a cause of the autoimmune process.
Wang J et al. Nutrients. 2015;7(4):2485-98. DOI: 10.3390/nu7042485 [Meta-analysis, 20 studies] · Jiang H et al. J Clin Pharm Ther. 2022;47(6):767-775. DOI: 10.1111/jcpt.13605 [Meta-analysis, 6 studies, n=258]Jill Hahn and colleagues from the Harvard T.H. Chan School of Public Health analyzed the VITAL trial. In it, 25,871 people, men aged 50 and older and women aged 55 and older, took 2,000 IU of vitamin D or a placebo daily for a median of 5.3 years.
On vitamin D, confirmed autoimmune diseases occurred in 123 people, on placebo in 155. That corresponded to a 22 percent lower rate. Autoimmune thyroid diseases were among the diagnoses recorded.
For you that means: vitamin D could steer the immune system in a calmer direction over the long term. The dose was a study dose in older adults and is not a recommendation for you.
Hahn J et al. BMJ. 2022;376:e066452. PMID: 35082139 · DOI: 10.1136/bmj-2021-066452 [RCT, n=25,871]How to put vitamin D into context and take it sensibly is described in Taking vitamin D correctly. Why levels drop so often in winter is explained in Vitamin D deficiency in winter. And what drives the immune system in Hashimoto's is explained in Hashimoto's: causes in the immune system.
Vitamin D is more conductor than musician. It does not play a single note of thyroid hormone itself. But it can influence how loudly the immune system plays in the orchestra.
And now you know why, in Hashimoto's, vitamin D is more a question of the immune system than of hormone values.
And iodine? The raw material, in good measure
Key point: Iodine is the raw material of the hormone, but too little and too much can both cause problems. According to a review, persistently too much iodine can promote autoimmune thyroiditis, and in a Belgian study with 1,241 pregnant women, free T4 depended more on iron than on mild iodine deficiency.
Maybe you have heard that you should take more iodine if you have hypothyroidism. Elsewhere you were told: with Hashimoto's, no iodine at all. Both sound certain. Both are too simple.
Hu and Rayman describe that persistently high iodine intake can trigger autoimmune thyroiditis, among other reasons because heavily iodinated thyroglobulin provokes the immune system more strongly. At the same time, iodine remains indispensable, and a true deficiency slows production.
For the chain, another point is decisive. In Côte d'Ivoire the iodine arrived, but without iron less came of it. In Belgium, the free T4 of pregnant women depended more on iron status than on mild iodine deficiency. So iodine is rarely the only bottleneck.
Iodine is like flour in a bakery. No flour, no bread. But without a working oven even the best flour stays raw dough, and in the thyroid, iron is part of that oven.
Before you add iodine, it is therefore worth looking at your intake and at iron and selenium. You will find the detailed context in Putting iodine in context and in Iodine in Hashimoto's.
And now you know why the question of iodine is always also a question of iron and selenium.
Which blood tests make the chain visible?
Key point: A single value only ever shows one link in the chain. Ferritin rises with inflammation, and selenium and zinc values in the blood were also clearly linked to CRP in a Glasgow cohort with 833 selenium and 743 zinc measurements.
Maybe you have already received a lab sheet with twenty values and still no answer. That is rarely due to the number of values. It is due to whether they are read together.
In the Glasgow analysis, the influence of inflammation on zinc could largely be adjusted for using the protein albumin, but not for selenium. So a low selenium value with a high CRP does not necessarily mean you are short of selenium.
Iron status
- Ferritin, never read alone
- Transferrin saturation
- Hemoglobin and blood count
- CRP as an inflammation marker
- if needed, soluble transferrin receptor
Thyroid
- TSH
- free T4
- free T3
- TPO and Tg antibodies
- if needed, ultrasound
Further building blocks
- 25-OH vitamin D
- Selenium, read with CRP
- Zinc, read with albumin and CRP
- Vitamin B12 and folic acid
- with unexplained iron deficiency, a celiac test and a look at the stomach
Which of these values make sense for you is decided by your doctor based on your history. More on thyroid values is in Thyroid values: which ones count, more on iron values in Iron deficiency: which blood values count. How silent inflammation can be measured is explained in Measuring silent inflammation.
When a closer look at your iron stores can be worthwhile
- You have Hashimoto's or hypothyroidism and are exhausted despite good values.
- Your periods are heavy or last a long time. More on this in Iron deficiency in women and menstruation.
- You eat a mostly vegetarian or vegan diet. More on this in Iron deficiency on a plant-based diet.
- You take acid blockers or often have stomach complaints.
- Your levothyroxine requirement is rising without a reason you know of.
- You get cold easily, are losing hair or have restless legs in the evening. More on this in Restless legs and iron.
- You are planning a pregnancy or are pregnant. Then your iron status belongs in your prenatal care with your gynecologist.
If you find yourself nodding at two or more points, that is no reason to worry. It is a good reason to ask specifically at your next appointment.
What the evidence supports and what remains open
Well supported
TPO is a heme enzyme, and deiodinases and glutathione peroxidases are selenoproteins. Iron and levothyroxine taken at the same time can reduce the effect of the tablet. Selenium can lower TPO antibodies in Hashimoto's.
Plausible, still thin in humans
That iron deficiency directly slows hormone production in humans. In the animal model this has been clearly shown, in humans mainly as an association. That zinc could support the conversion of T4 to T3.
Open and debated
Which ferritin value is the right one: the answers range from 15 through 25 and 50 to 100. Whether vitamin D changes thyroid function or mainly the antibodies, and whether a low level is cause or consequence.
Clinical observation
My ferritin target above 100 when there are symptoms, and the pattern from the story. The observation that symptoms eased in two thirds of women once their ferritin was above 100 also does not come from a randomized trial.
- Have the chain measured, not just one link. Ferritin always together with transferrin saturation, CRP and a blood count, plus TSH, free T4, free T3 and the antibodies. Ask to be given the numbers, not just the word unremarkable.
- Look for the cause before you refill. Ask why a store is empty: bleeding, the stomach, celiac disease or diet. Only then does it become clear whether tablets are enough or whether a correctly performed infusion may make more sense.
- Supplement in a targeted way, not by gut feeling. Selenium, zinc and iodine only with a documented need and with an end date. Take iron and levothyroxine at separate times and discuss the interval with your doctor. Never change your dose on your own.
The best question about the thyroid and nutrients is not: what else should I take? It is: which link in my chain is the shortest right now, and how will I know when it is enough again?
In my clinical practice I observe that this very question often changes more than any additional jar in the cupboard. That is experience from my consultations, not a study result.
And now you know why your thyroid sometimes does not need more hormone, but a fuller set of tools.
Frequently asked questions about nutrients for the thyroid
Which nutrients does the thyroid need most?
Above all, the thyroid needs iodine, iron, selenium and zinc, plus vitamin D for the immune system. Iodine is the raw material of the hormone. Iron sits at the center of thyroid peroxidase, the enzyme that builds the hormone. Selenium protects the gland and is part of the deiodinases, which convert T4 into active T3. Zinc is involved in control hormones and in the T3 receptor. Only a measurement can show which building block you are missing, because with selenium, zinc and iodine, too much can cause problems just as too little can.
Can iron deficiency worsen hypothyroidism?
There is a lot to suggest it, but it has not been proven in humans. Thyroid peroxidase is a heme enzyme, and in rats fed a low-iron diet its activity fell by up to 56 percent. In a meta-analysis of ten studies, free T4 was on average 1.18 pmol/L lower with iron deficiency. In women of reproductive age, iron deficiency was associated with 1.89 times the odds of TPO antibodies. These associations make a medical check of your iron status sensible if you have symptoms of an underactive thyroid.
What ferritin level is good for the thyroid?
There is no official target for the thyroid. When someone has symptoms, I aim for a ferritin above 100 µg/L. That is my clinical position, not a general guideline recommendation. It rests on isotope studies showing that the body already absorbs more iron below about 50 µg/L, and on an analysis of 29 guidelines that recommends 100 µg/L for diagnosis in most situations. A review also describes that symptoms eased in two thirds of women on levothyroxine once their ferritin was above 100. Ferritin should always be read together with transferrin saturation and CRP.
Can I have iron deficiency even though my blood count is normal?
Yes. The stores empty long before hemoglobin falls. In a multinational analysis, hemoglobin in women only began to fall below a ferritin of about 25 µg/L. In a randomized trial with 198 tired women without anemia and with a ferritin below 50 µg/L, fatigue decreased by 47.7 percent on iron and by 28.8 percent on placebo. An unremarkable blood count therefore does not rule out iron deficiency.
What is functional iron deficiency?
Functional iron deficiency means that iron is stored but does not reach the places where it is needed. During inflammation, the hormone hepcidin rises, which slows iron absorption and holds iron in the stores. At the same time, ferritin rises as an inflammation protein and can mask a deficiency. That is why transferrin saturation and CRP belong in the assessment. In Hashimoto's the immune system is active, but how strongly that alone distorts ferritin has been little studied.
Can I take levothyroxine and iron together?
Not at the same time, and the right interval is best discussed with your doctor. In a study of 14 people whose treatment was well adjusted, mean TSH rose from 1.6 to 5.4 mU/L when they took iron at the same time as levothyroxine for twelve weeks. Iron binds the hormone and forms a poorly soluble complex. When you start or stop iron, a TSH check after a few weeks can make sense. Please never change your dose on your own.
Can hypothyroidism itself lead to iron deficiency?
Yes, that is possible. Thyroid hormones stimulate blood formation, and hypothyroidism can dampen it. In a study from Thessaloniki, 23.4 percent of women with hypothyroidism had irregular bleeding, often very heavy, compared with 8.4 percent of the comparison women. In Hashimoto's, autoimmune gastritis is often added, which lowers stomach acid and with it iron absorption. This can create a cycle in which hypothyroidism and iron deficiency reinforce each other.
Do I need selenium if I have Hashimoto's?
Not automatically. It makes sense mainly when a need has been shown. A meta-analysis of 35 studies found lower TPO antibodies on selenium, while free T4 and free T3 did not change significantly. The European Food Safety Authority sets the upper limit for adults at 255 micrograms per day from all sources. In a large trial, type 2 diabetes occurred more often on 200 micrograms of selenium daily. That is why, with selenium, measuring comes before taking.
What does zinc do for the thyroid?
Zinc is involved at several switch points, but the evidence on zinc supplements is thin. According to a review, zinc influences the deiodinases, the production of TRH and TSH, and transcription factors involved in hormone production. In a small randomized trial with 68 women with hypothyroidism, free T3 rose in the zinc groups, but most differences between the groups were not significant. Too much zinc can cause anemia through copper deficiency. Zinc should therefore be supplemented in a targeted way, not as a precaution.
Does vitamin D lower Hashimoto's antibodies?
Studies suggest that vitamin D can lower TPO antibodies, while hormone values changed little. A meta-analysis with 258 people with Hashimoto's found lower TPO antibodies, but no significant differences in TSH, free T3 and free T4. In the VITAL trial with 25,871 older adults, 22 percent fewer autoimmune diseases occurred on vitamin D over about five years. Whether a low vitamin D level is a cause or a consequence of the autoimmune process is considered open.
Should I take more iodine if I have hypothyroidism?
Not without a proper workup, because too little and too much iodine can both cause problems. According to a review, persistently too much iodine can promote autoimmune thyroiditis. In children with goiter in Côte d'Ivoire, iodine had clearly less effect when iron deficiency was present, and in pregnant women in Belgium, free T4 depended more on iron status than on mild iodine deficiency. Whether you need iodine and how much depends on your intake and your diagnosis, and belongs in a conversation with your doctor.
Can an iron infusion make sense for people with thyroid conditions?
It can be a good option when tablets are not enough and the infusion is done correctly. That includes a clear indication, ruling out iron overload such as in hemochromatosis, a modern preparation and good monitoring. A 2021 Lancet review describes that modern preparations allow the total dose to be given quickly and safely. The poor reputation of infusions stems largely from the era of older, high-molecular-weight preparations. Whether it suits you is an individual medical decision.
Which blood tests should I have done if I have thyroid symptoms?
It makes sense to look at the whole chain rather than at a single value. For the thyroid, that means TSH, free T4, free T3 and the antibodies against TPO and thyroglobulin. For iron, it means ferritin, transferrin saturation, a blood count and CRP. Depending on the situation, vitamin D, selenium, zinc, vitamin B12 and, if an iron deficiency is unexplained, a celiac test can be added. Your doctor decides which values fit your situation.
Where the topic continues
Iron deficiency and iron infusions
The big overview of stores, values and infusions
FerritinFerritin above 100: a sensible target?
The detailed reasoning behind my clinical position
FunctionalFunctional iron deficiency
When iron is stored but does not arrive
EnergyIron deficiency, tiredness and exhaustion
Why empty stores often show up first as a loss of energy
InfusionChecklist for a well-done iron infusion
How you can recognize a careful approach
DetoxificationGlutathione
The partner of the selenium fire extinguisher in the thyroid
FastingIntermittent fasting and iron deficiency
Why a small eating window can strain empty stores
DiagnosticsWhole blood or serum?
What a micronutrient analysis shows and what it misses
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