Fasting with Hashimoto's: relief for the thyroid or stress on the axis?
The answer is not yes and not no. It depends on how much inflammation your immune system is carrying and how much reserve your stress system still has.
Fasting is not a medication and not an enemy. It is an announced withdrawal of energy. And whether your body answers it with tidying up or with saving mode is decided not by the trend, but by the state you were in the day before.
I bet you know this feeling. You have Hashimoto's, you have been reading through everything you can find for months, and at some point you stumble over fasting. One podcast says fasting turns autoimmunity down. One forum says fasting has ruined the thyroid for good. Both sound convinced. Both sound experienced. And you stand in between and do not know whom to believe.
Then you may ask at your practice. And often there is simply no time for this question, because a short appointment for history, values and adjustment is tightly measured anyway. What is missing then is not competence, but room for an explanation.
That is what I want to change here. Not by telling you what is right. But by showing you which two forces run at the same time during fasting. One of them may do you good. The other may harm you. Which one wins depends on the state you start from.
What awaits you in this article
- Why fasting triggers two opposing effects at the same time
- What actually happens in the immune system in Hashimoto's
- How quickly and why T3 falls without food
- What cortisol does and why duration decides
- Why women react differently than men
- The decision matrix of inflammatory load and stress load
- Selenium, iron, vitamin D and iodine as quiet players
- Levothyroxine in the fasting window and the Ramadan data
- Refeeding: why the exit is often riskier than the entry
- Who should not fast, no ifs and no buts
The question is not whether fasting is good. It is what it costs.
Imagine your immune system as a fire brigade that has been working a false alarm for years. It is on permanent duty in a place where there is no fire: at your thyroid. That is Hashimoto's in one image.
And now imagine your metabolic system as a power grid. The thyroid is the dial that determines how much current flows into every single cell. Warmth, heartbeat, digestion, concentration, cycle, hair. Everything hangs on that dial.
Fasting does something to both systems. It may calm the fire brigade. And at the same time it turns the dial down. That is not a contradiction. That is exactly the point where most guides stop too early.
Fasting may dampen inflammatory signals and at the same time lowers energy availability. With a healthy thyroid that is a stimulus. With a thyroid under autoimmune attack it can be a bill that you cannot always afford.
That is why I put the question differently. Not: is fasting good or bad in Hashimoto's? But: how much inflammatory load are you carrying right now, and how much reserve does your stress system still have? Out of those two values comes a surprisingly clear answer. And now you know why a blanket recommendation is worth nothing here.
What really happens in the body in Hashimoto's
Before we talk about fasting, we need to know what it meets. And there is a misunderstanding here that I hear almost every week: that Hashimoto's is a disease of the thyroid. It is not. It is a disease of the immune system that plays out at the thyroid.
In Hashimoto's thyroiditis the immune system loses tolerance towards its own tissue. T lymphocytes migrate into the thyroid, the tissue is remodelled over years, scarring and loss of functional tissue develop. This becomes detectable through antibodies against thyroid peroxidase and against thyroglobulin.
Weetman of the University of Sheffield summarised the state of research on how Hashimoto's develops. He describes several cytokine networks in which the thyroid cells themselves become amplifiers of the inflammation, as well as a whole range of different defects in regulatory T cells.
What that means for you: regulatory T cells are the brake of your immune system. When this brake weakens, the inflammation runs on even when no pathogen is there any more. Anything that supports this brake is theoretically interesting. And this is exactly where the fasting hypothesis comes in.
Weetman AP. An update on the pathogenesis of Hashimoto's thyroiditis. J Endocrinol Invest. 2021;44(5):883-890. DOI: 10.1007/s40618-020-01477-1The condition is common and it affects women far more often than men. In regions with sufficient iodine supply it is the leading cause of an underactive thyroid. And it rarely comes alone: in roughly one fifth of cases further autoimmune conditions are involved.
Hypothalamus
Reports the body's energy status and sends out TRH. It is the first to notice when food does not arrive.
Pituitary gland
Answers with TSH. That is the value almost every practice measures, and it shows only one section of the picture.
Thyroid
Produces mainly T4, the storage hormone. In Hashimoto's it works with reduced tissue.
Cell and deiodinases
This is where everything is decided: T4 is converted either into the active T3 or into the inactive reverse T3. Fasting intervenes at exactly this point.
Your TSH is not your thyroid function. TSH is the order your brain places. What arrives in your cells is written somewhere else: in free T3 and in what your deiodinases make of it.
That is why someone with a pretty TSH of 1.8 can feel wretched. And that is why the fasting question is a question aimed at the fourth step of the axis, not the second.
And now you know why fasting has to be judged differently in Hashimoto's than in a person with a healthy gland. You have less buffer. Not none. Less.
What fasting may do to the inflammation
Let us start with the good side. Because it is real, and I do not want to make it smaller than it is.
When food does not arrive, metabolism switches over. Insulin falls, ketone bodies rise, and signalling pathways such as mTOR wind down. That is the state in which cells tidy up: autophagy. Damaged components are broken down and recycled. For an immune system that constantly reads cell debris as an alarm signal, that is an exciting thought.
A working group around Choi at the University of Southern California, which also involved researchers from the Charité in Berlin, investigated a fasting mimicking diet in the mouse model of multiple sclerosis. Three days of this diet, repeated in cycles, reduced the severity of the condition in all animals, and in 20 percent of the animals the symptoms receded completely.
What was observed: more regulatory T cells, fewer inflammation promoting cytokines, fewer TH1 and TH17 cells. And one detail I do not want to leave out, because it is uncomfortable: in this work the improvement was also associated with raised corticosterone levels, meaning the animal equivalent of exactly the cortisol rise I describe further below as the price of fasting. In addition the authors reported preliminary data from a small investigation in people with relapsing multiple sclerosis, in which the diet was judged feasible and safe.
What that means for you: the mechanism is biologically plausible and shown in an autoimmune model. In humans, and specifically in Hashimoto's, it is not proven. A mouse is not a woman with Hashimoto's.
Choi IY, Piccio L, Childress P et al. A Diet Mimicking Fasting Promotes Regeneration and Reduces Autoimmunity and Multiple Sclerosis Symptoms. Cell Rep. 2016;15(10):2136-2146. DOI: 10.1016/j.celrep.2016.05.009The second trail leads to another autoimmune condition where controlled human data actually exist: rheumatoid arthritis.
Müller and colleagues screened 31 reports on fasting studies in rheumatoid arthritis. Only four controlled investigations had followed up participants for at least three months. The pooled analysis of these four studies showed a statistically and clinically meaningful long term effect in favour of fasting followed by a vegetarian diet.
What that means for you: in an inflammatory autoimmune condition, fasting in combination with a subsequent plant focused diet may apparently move something. The authors themselves called for more and better studies. And rheumatoid arthritis is not Hashimoto's: in rheumatoid arthritis pain is in the foreground, in Hashimoto's it is the energy supply of the whole body.
Müller H, de Toledo FW, Resch KL. Fasting followed by vegetarian diet in patients with rheumatoid arthritis: a systematic review. Scand J Rheumatol. 2001;30(1):1-10. DOI: 10.1080/030097401750065256An Australian group around Chaudhary tested whether intermittent fasting activates autophagy markers. In the mouse liver the markers rose under intermittent fasting. In the skeletal muscle of the mice they did not. And in 50 women who fasted for 24 hours on three non consecutive days per week over eight weeks the picture was mixed: after 24 hours of fasting one marker rose, while in the group that additionally took in less energy several markers were lower after a twelve hour overnight fast. The authors attribute this more to the weight loss than to the fasting pattern itself.
What that means for you: autophagy is not a light switch that 16 hours without food flips reliably. It is tissue dependent, hard to measure and in humans far less predictable than the popular account suggests.
Chaudhary R, Liu B, Bensalem J et al. Intermittent fasting activates markers of autophagy in mouse liver, but not muscle from mouse or humans. Nutrition. 2022;101:111662. DOI: 10.1016/j.nut.2022.111662To this day there is no good study that has measured TPO antibodies in people with Hashimoto's under fasting. Anyone who tells you that fasting lowers your antibodies is telling you a hypothesis dressed up as a fact.
That does not make the hypothesis wrong. It only makes it what it is: a reasoned assumption that is worth researching.
And now you know why I am cautiously optimistic on the topic of inflammation and fasting, but never categorical. The direction fits. The proof for your diagnosis is missing.
The price: why T3 falls during fasting
Now to the other side. And it is better documented than the first.
When your body notices that food is not arriving, it does something very clever. It lowers its consumption. And the central lever for that is the thyroid axis. It slows the conversion of T4 into the active T3 and diverts more T4 into the inactive reverse T3. The result is a pattern that in technical language is called low T3 syndrome or nonthyroidal illness syndrome.
Farwell summarised the physiology of this pattern. Involved are changes in deiodinase activity of type 1 and 3, an altered release of TRH and TSH, altered binding to transport proteins in the blood, as well as shifts in the transporters that bring thyroid hormone into the cell in the first place.
What that means for you: the braking does not happen at one place, but at four at once. That is why one look at TSH is not enough to see whether your body is currently saving.
Farwell AP. Nonthyroidal illness syndrome. Curr Opin Endocrinol Diabetes Obes. 2013;20(5):478-484. DOI: 10.1097/01.med.0000433069.09294.e8This is not only theory. It can be measured in healthy people who fast voluntarily.
A Graz group around Stekovic investigated four weeks of strict every other day fasting in healthy, non obese middle aged people in a randomised controlled trial. On average this produced a calorie reduction of 37 percent. The study found favourable effects on cardiovascular markers, fat mass and inflammatory markers.
And it found something that is central for you: in people who had practised this form of fasting over a longer period, alongside the inflammatory marker sICAM-1 and LDL, triiodothyronine was also lower, meaning exactly the active thyroid hormone. Important for the context: this finding comes from the cross-sectional comparison of the long term practitioners within the work, not from the four week randomised arm. No cause and effect conclusion can be drawn from it.
What that means for you: even when a fasting protocol looks advantageous overall, it does not pass the thyroid axis without a trace. In healthy people this is an adaptation. In you it lands on a system that is already working at a reduced level.
Stekovic S, Hofer SJ, Tripolt N et al. Alternate Day Fasting Improves Physiological and Molecular Markers of Aging in Healthy, Non-obese Humans. Cell Metab. 2019;30(3):462-476. DOI: 10.1016/j.cmet.2019.07.016Moro and colleagues followed 34 resistance trained men over eight weeks. One group ate their entire energy intake within an eight hour window, the other spread it across the day. Both groups received the same number of calories and the same nutrient distribution.
Fat mass fell more strongly in the fasting group, muscle mass and maximal strength were maintained. At the same time testosterone and the growth factor IGF-1 fell significantly in the fasting group, and triiodothyronine went down as well. TSH did not change.
What that means for you: here it was not down to the amount of calories, because that was the same. It was down to the timing pattern. And one more thing stands out: TSH stayed unchanged while T3 fell. Anyone who only checks TSH does not see this shift.
Moro T, Tinsley G, Bianco A et al. Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males. J Transl Med. 2016;14(1):290. DOI: 10.1186/s12967-016-1044-0A falling T3 is not damage. It is an answer. Your body says: I currently have little, so I use little. In a healthy person that is elegant.
In Hashimoto's it is a second throttling on top of a first one. And that is why you may feel it earlier: in hands that stay cold. In hair that lies in the drain. In a tiredness that does not improve after eating. This is not documented for Hashimoto's, I am deriving it from physiology.
And now you know why the length of the fast is the decisive adjusting screw. Not the question of whether, but the question of how long.
Cortisol: the second price that hardly anyone counts in
There is a sentence I often hear in the practice. It goes: while fasting my head feels completely clear, only afterwards do I fall into a hole. And that is exactly the point where cortisol comes into play.
For the body, fasting is a stress stimulus. Not in the moral sense, but in the biological one. And the stress system answers the way it always answers: with cortisol. That explains the clarity in the head. And it explains why some people experience exactly the opposite after a few weeks.
Nakamura and colleagues analysed 13 studies with a total of 357 participants in which cortisol was measured after calorie reduction. Across all studies, serum cortisol rose significantly. With actual fasting the effect was very strong. With very low calorie and with moderately calorie reduced diets, by contrast, the rise was not significant.
The meta-regression showed a further important point: the cortisol rise occurred mainly in the initial phase and moved back towards the starting value after a few weeks.
What that means for you: the dose decides. A mild extension of the overnight break is something entirely different from actual food abstinence. And if your stress system is already running at its limit, long fasting adds stress on top of stress.
Nakamura Y, Walker BR, Ikuta T. Systematic review and meta-analysis reveals acutely elevated plasma cortisol following fasting but not less severe calorie restriction. Stress. 2016;19(2):151-157. DOI: 10.3109/10253890.2015.1121984Why is this particularly relevant in Hashimoto's? Because cortisol and the thyroid are closely coupled. From physiology it is known that glucocorticoids can act on the deiodinases and on TSH release. A persistently high cortisol load could therefore additionally brake the conversion of T4 into T3. I do not know of robust studies on this specifically in Hashimoto's. So a throttling from two sides is conceivable: through the energy shortage itself and through the stress response to it.
"Fasting is a stimulus. A stimulus needs a system that is capable of recovery. Whoever fasts while exhausted is not training. They are draining."
Shukri Jarmoukli, ViveCura BerlinAnd now you know why in Hashimoto's I ask about sleep first and not about the eating window. A person who lies awake at three in the morning does not need another stress stimulus.
Why women react differently than men
May I ask you an uncomfortable question? Did you take over your fasting protocol from a man?
That is not a dig. It is a seriously meant question. Because a large part of the popular fasting protocols comes from studies in men or from the experience of men. The Moro study on the 16:8 rhythm, for example, was carried out exclusively in men.
The female body, however, has an additional early warning system. The cycle is closely tied to energy availability. When the brain registers an energy shortage, it can wind down the reproductive axis long before any lab value becomes conspicuous. The Endocrine Society describes exactly this connection in its guideline on functional hypothalamic amenorrhoea: stress, weight loss and heavy physical strain can bring cycle regulation to a standstill. Cycle shifts, absent ovulation, shorter luteal phases: these are first signals, not footnotes.
What I pay particular attention to
- Hashimoto's affects women far more often than men
- Iron deficiency through menstruation is common and often unrecognised
- The cycle reacts early to an energy deficit
- Sleep quality often tips before the lab values do
- Hair loss as an early signal of a throttling
What I also pay attention to
- Testosterone and IGF-1 fell significantly in the 16:8 study
- Training performance can be the first thing to give way
- Drive and libido as sensitive markers
- Muscle mass was maintained with sufficient protein
- Fasting protocols are more often overstretched
A Chinese group around Li followed 18 young women with polycystic ovary syndrome and absent ovulation, 15 of whom completed the investigation. Five weeks with an eight hour eating window went along with better values for insulin resistance, inflammation and androgens. In 73.3 percent, cycle regularity improved. At the same time body weight, BMI and body fat fell markedly. Part of the hormonal improvement could therefore be down to the weight loss and not to the time window.
What that means for you: a shorter eating window is not automatically harmful for women. With an insulin resistant starting position it may be favourable. The difference lies not in sex alone, but in the metabolic situation. And the group was small and without a control group. That is a hint, not a proof.
Li C, Xing C, Zhang J et al. Eight-hour time-restricted feeding improves endocrine and metabolic profiles in women with anovulatory polycystic ovary syndrome. J Transl Med. 2021;19(1):148. DOI: 10.1186/s12967-021-02817-2This is not about whether women are allowed to fast. It is about the fact that the female body reports earlier when the sums do not add up. That is not a disadvantage. That is a better warning system.
If your cycle changes, that is not a side issue to be sat out. It is the first lab value you can read without a blood draw.
And now you know why I start differently with women who have Hashimoto's than with men. Not more anxiously. More precisely.
The decision line: inflammatory load against stress load
Now comes the part you are probably reading this article for. How do I decide whether fasting fits me?
I think in two axes. One is the inflammatory load: how active is your immune system right now? The other is the stress load: how resilient is your nervous system right now, how stable are sleep, blood pressure and recovery after exertion? By this I explicitly do not mean adrenal fatigue in the popular sense. That concept is not scientifically established. A genuine adrenal insufficiency, by contrast, is a rare but serious condition that belongs in medical diagnosis.
Out of these two axes a field with four rooms emerges. This is not a diagnostic instrument. It is a map for a conversation.
High inflammation, good stress reserve
You have raised antibodies, perhaps joint or skin complaints, but you sleep well, your weight is stable and you recover briskly after exertion. Here a mild form of fasting is most likely worth a supervised attempt. Short windows, no heroics.
High inflammation, low stress reserve
This is the combination I see most often. Antibodies high, exhaustion high, sleep torn apart. Here fasting may deepen the exhaustion before it benefits the inflammation. First stabilisation, then perhaps fasting at some point.
Low inflammation, good stress reserve
Values stable, symptoms quiet, everyday life carries. Here it is usually not about therapy at all, but about prevention and metabolic health. A calm overnight break is usually manageable here, provided the medication intake is safely in place and you pay attention to your own signals.
Low inflammation, low stress reserve
The thyroid is quiet, but you are not. Often something else sits behind this: a sleep disorder, iron deficiency, vitamin D deficiency, emotional strain. Fasting here would address a problem that is not the problem.
Most people ask: how long am I allowed to fast? The better question is: what needs to be stable for fasting to become a sensible question at all?
Sleep, iron, vitamin D, protein, blood sugar calm. When these five are in place, fasting is a tool. When they are not, fasting is a distraction.
And now you know why my most honest answer to the fasting question often begins with a question back.
The quiet players: selenium, iron, vitamin D and iodine
There is a reason why I so often come to talk about micronutrients when it comes to fasting. Fasting does not only mean less energy. It also means less intake of exactly the building blocks your thyroid needs in order to work.
Selenium: the gland's protective shield
During hormone production the thyroid generates hydrogen peroxide. That is necessary and aggressive at the same time. Selenium dependent enzymes, the glutathione peroxidases, clear away the excess. If selenium is missing, more oxidative stress may remain standing in the tissue.
A Bern group around Huwiler systematically analysed randomised trials on selenium in Hashimoto's. Out of 687 screened works, 35 studies remained. In people without thyroid hormone replacement, TSH fell slightly under selenium, with a standardised mean difference of minus 0.21. TPO antibodies fell more clearly, at minus 0.96, and did so independently of whether levothyroxine was being taken.
Side effects in the studies analysed did not differ from the control group. The authors rated the certainty of the evidence overall as moderate, and the scatter between the studies was very high for the antibody result.
What that means for you: selenium is one of the few building blocks with a decent data situation in Hashimoto's. It is not a substitute for thyroid hormone and not a treatment in the narrow sense. And more is not better: selenium has a narrow safety range. The European Food Safety Authority names 255 micrograms per day as the tolerable upper intake level for adults. Permanently above that, a selenium overload can develop, with hair loss, brittle nails, gastrointestinal complaints and nerve irritation. So the rule is: measure first, give in a targeted way, set a time limit.
Huwiler VV, Maissen-Abgottspon S, Stanga Z et al. Selenium Supplementation in Patients with Hashimoto Thyroiditis: A Systematic Review and Meta-Analysis of Randomized Clinical Trials. Thyroid. 2024;34(3):295-313. DOI: 10.1089/thy.2023.0556A Cochrane review by van Zuuren and colleagues came to a more reserved conclusion as early as 2013. On the basis of four studies with 463 participants it stated that the evidence was sufficient neither to confirm nor to refute. A later overview of six systematic reviews with 75 randomised trials did find a reduction in antibodies after three and six months, but rated the certainty of the evidence as low to very low. I consider selenium sensibly examined. I do not consider it proven.
Iron: the overlooked bottleneck
This is where it becomes particularly important for women with Hashimoto's. Thyroid peroxidase, meaning exactly the enzyme your antibodies are directed against, is a haem enzyme. It only becomes active once it has bound haem. And haem needs iron.
Rayman of the University of Surrey summarised the role of iodine, iron and selenium in autoimmune thyroid disease. Iron deficiency impairs thyroid metabolism, because thyroid peroxidase cannot work without bound haem. People with autoimmune thyroiditis are frequently low in iron, among other reasons because an autoimmune gastritis hinders iron absorption and coeliac disease can lead to losses.
It is also reported, as an observation referred to within the review rather than an original finding of it, that in two thirds of women with persistent hypothyroid symptoms despite appropriate levothyroxine therapy, the symptoms improved when serum ferritin was raised above 100 µg/L.
What that means for you: if you feel unwell despite a well adjusted TSH, ferritin is one of the first values I would look at. And anyone who fasts with empty iron stores feels every throttling twice over.
Rayman MP. Multiple nutritional factors and thyroid disease, with particular reference to autoimmune thyroid disease. Proc Nutr Soc. 2019;78(1):34-44. DOI: 10.1017/S0029665118001192Vitamin D and iodine: two values with different logic
Vitamin D
A review by Durá-Travé and Gallinas-Victoriano describes that vitamin D levels in Hashimoto's are consistently lower than in healthy people, and that after giving cholecalciferol in proven deficiency the antibody titres were lower in several investigations. This is a summarising review, not a controlled intervention study, and the authors explicitly call for further randomised studies. In practice that means: measure first, then top up in a targeted way, do not dose high across the board. Too much vitamin D over a longer period can send the calcium level off course. A review by Hu and Rayman adds the consideration that a low vitamin D level could also be a consequence of the autoimmune process itself and not only its cause.
Iodine
With iodine a different logic applies than with the other building blocks: more is not better here. According to the review by Hu and Rayman, a chronically excessive iodine intake can favour an autoimmune thyroiditis, among other reasons because heavily iodinated thyroglobulin is more conspicuous to the immune system. Nationwide salt iodisation was also associated with a temporary rise in some regions. High dose iodine preparations therefore do not belong in self medication in Hashimoto's.
Whoever fasts does not only eat fewer calories. They also eat less selenium, less iron, less zinc, less protein. In a well supplied person that is meaningless over a few hours.
In a person with empty stores it is the difference between a stimulus and a deficiency. That is why the stores come first and the eating window afterwards.
And now you know why the lab comes before the calendar.
Levothyroxine in the fasting window: the most practical point of all
This section is unspectacular and nevertheless the one that most often makes the difference in practice.
Levothyroxine is a prescription-only thyroid hormone. It is dosed individually by a doctor and monitored through blood values, and there is a reason for that. An adjustment set too high can over time favour cardiac arrhythmias up to atrial fibrillation and put strain on bone density. One set too low brings the hypothyroid symptoms back. Particular caution applies in coronary heart disease, in cardiac arrhythmias and in older people. That is why any change of dose belongs in medical hands, never in solo experiments.
Levothyroxine is best absorbed on an empty stomach, at a distance from coffee, dairy products, calcium and iron preparations. Acid blockers, colestyramine, sevelamer and antacids can also interfere with absorption. A fasting window can even make this fasted phase easier. The problem is not the empty stomach. The problem is the shifted routine.
And for that there are excellent data from a natural experiment: Ramadan. Millions of people change their eating times over weeks and with them their intake times.
Alkaf and colleagues analysed thyroid values of 481 people with primary hypothyroidism on levothyroxine, before and after Ramadan in each case. The median TSH rose from 2.0 to 2.9 µIU/mL, free T4 and free T3 fell. 25.5 percent of those previously well adjusted were no longer in the target range afterwards. A higher baseline TSH was the strongest predictor. After three to six months the values had settled again.
What that means for you: for the majority the shift is mild and temporary. For a quarter it is not. Anyone who is only just adjusted belongs in this risk group.
Alkaf B, Siddiqui M, Ali T et al. Ramadan Fasting and Changes in Thyroid Function in Hypothyroidism: Identifying Patients at Risk. Thyroid. 2022;32(4):368-375. DOI: 10.1089/thy.2021.0512Belal and colleagues summarised 14 studies, three of them randomised. Result: TSH rose significantly after Ramadan, while free T4 remained stable. And particularly practical: none of the intake times investigated was superior to the others. Neither before breaking the fast, nor after it, nor before the morning meal.
What that means for you: there is no one right time of day. There is the time of day that you keep to reliably. Reliability beats optimisation.
Belal MM, Youssef AR, Baker H et al. Effect of Ramadan fasting on thyroid functions in hypothyroid patients taking levothyroxine: a systematic review and meta-analysis. Ir J Med Sci. 2024;193(2):741-753. DOI: 10.1007/s11845-023-03526-zIn a Turkish investigation by Dellal and colleagues in 62 treated people, the median TSH did not rise significantly during Ramadan. Something else was striking, however: free T3 fell while free T4 rose. This is exactly the pattern you now know. The store fills up, the conversion into the active form brakes. Anyone who measures only TSH sees none of this.
And now you know why the most boring point of this article may be the most important one.
The exit: why refeeding is often riskier than going without
Much is written about starting a fast. About ending one, almost nothing. Yet that is where the actual medical risk sits.
After a longer period without food the body switches to fat burning and empties quiet stores in the process, above all of phosphate, potassium, magnesium and thiamine. When carbohydrates then arrive again, the pancreas releases insulin. And insulin pushes exactly these minerals abruptly out of the blood and into the cells. That is the core of refeeding syndrome. It can put strain on heart rhythm, muscles and the nervous system.
With an extended overnight break a refeeding syndrome is not to be expected. The risk rises with the duration of the abstinence and with the starting condition. Particularly at risk are people who are underweight, who have had a long period of low food intake, who have a history of alcohol use and who have eating disorders. In the specialist literature on anorexia in adolescence, the refeeding risk is one of the central safety questions of all.
Two further points belong here with longer fasting. Rapid weight loss can trigger an attack of gout and favour the formation of gallstones. Neither is a reason to panic, but both are a reason not to plan longer fasting on your own.
Fasting is often told as a feat of discipline. The medically demanding part, however, is not the leaving out. It is the coming back.
Anyone who fasts for several days should plan the return to eating just as seriously as the abstinence. And do so with support, not by gut feeling.
And now you know why with multi-day fasting and an autoimmune condition I always ask about medical supervision. Not out of caution for caution's sake. But because the exit needs expertise.
Who is better off not fasting
There are situations in which I do not weigh things up. Not because I want to be strict, but because the ratio of possible benefit to possible harm is unambiguous here.
Clear exclusion situations
- Trying to conceive, pregnancy and breastfeeding. During this time the thyroid demand rises markedly, and the development of the child depends directly on the mother's supply. A systematic review of 43 studies on Ramadan fasting in pregnancy found no strong evidence of harm overall, but a weak data situation and, in one of the few high quality studies, a lower birth weight. Individual studies of medium quality also found unfavourable effects on fetal growth.
- Eating disorder in the past or currently. Fasting can reactivate restrictive patterns. Here the psychological risk outweighs any possible metabolic benefit. If you notice that your relationship with food is weighing on you, that is not a question of character but a reason to get support. In Germany the BZgA eating disorder helpline advises anonymously and free of charge on 0221 892031. In an acute crisis the Telefonseelsorge is reachable around the clock on 0800 111 0 111, and in an emergency the number is 112.
- Underweight or unintended weight loss. Anyone already losing substance needs intake, not abstinence.
- Children and adolescents. Growth and fasting do not go together.
- Newly adjusted or fluctuating thyroid values. First stabilise the adjustment, then everything else.
- Diabetes with blood sugar lowering medication. Only with medical adjustment, because of the risk of hypoglycaemia.
- Pronounced exhaustion with disturbed sleep. Fasting here would amplify the existing stress rather than reduce it.
- Known or suspected adrenal insufficiency. In Addison's disease and related pictures, going without food can trigger a dangerous crisis. That belongs clarified beforehand.
- Advanced kidney or liver disease. Both organs help steer metabolism during fasting. Without medical clarification this is no field for solo experiments.
- Other long term medication. Blood pressure lowering and water tablets can tip circulation and electrolytes during fasting. Metformin and blood thinning medication whose effect is co-determined by diet also belong discussed beforehand.
What I observe in practice
I am deliberately describing a general pattern here and not a case history. No names, no individual stories, no values from a particular file.
What strikes me again and again in practice
People with Hashimoto's often come into the consultation with a fasting experience they have already had. And over the years one observation repeats itself, which I can describe as a pattern, not as evidence.
The pattern goes like this: many people describe a mild form of a break from eating to me as manageable and pleasant. That is my impression from conversations, not a survey, and there are people for whom even that does not agree. If it becomes a distinctly stricter form, it tips into exhaustion in a proportion of people. Frequently together with feeling cold, hair loss and a sleep that breaks off in the early morning hours. Not in everyone. But in enough people for me to notice it.
What I regularly encounter when I look more closely: empty iron stores, a low vitamin D level and a sleep quality that was already poor before the fasting. So exactly the three things that form the buffer.
I cannot claim any causality here. I am merely documenting a temporal association that comes up repeatedly in conversations. There may be other explanations, and to this day there is no study that has examined this pattern.
The lesson from it: in Hashimoto's it is not the idea that decides the benefit, but the dose and the state you start from.
Many people with Hashimoto's believe they have to be harder on themselves in order to get healthier. Eat more strictly, fast for longer, more discipline.
With an autoimmune condition the opposite is often the way. Your immune system is already in a state of alarm. It does not need an additional signal of scarcity. It needs a signal of safety.
Three levers you can touch this week
No protocols, no rows of numbers. Three directions that make sense for most people with Hashimoto's, regardless of whether you end up fasting or not.
The night before the eating window
Before you adjust the eating window, adjust your sleep. A calm, dark, sufficiently long night can markedly relieve the stress pressure on the axis. In my view this is the site worth working on first. When your sleep is in place, you have a basis on which a fasting stimulus can work as a stimulus in the first place, rather than as a drain.
The stores before the break
Have ferritin, vitamin D, vitamin B12 and a blood count measured, plus free T3 and free T4 and not only TSH. If these values are weak, filling them up is the first step and fasting the second at the earliest. The order matters more here than the selection.
The mild form first
If you want to try it, begin with the gentlest variant that changes anything at all: a calm, reliable break from eating overnight, without snacks late in the evening. Observe body temperature, sleep, cycle, hair and energy for four weeks. If these five stay stable, you have an answer. If they do not, you have one as well.
Self check: is now the right time for fasting?
- My thyroid values have been stable for at least a few months.
- I sleep through most nights and do not regularly wake between three and four in the morning.
- My weight is stable, I am not losing weight unintentionally.
- My ferritin and my vitamin D level are known and not on the floor.
- I am not trying to conceive, not pregnant and not breastfeeding.
- I have never had an eating disorder and my relationship with food is relaxed.
- I know when I take my levothyroxine, and I keep to that time reliably.
- I have discussed all other conditions and long term medications medically, in particular heart, kidneys, liver, blood pressure, diabetes and adrenal function.
- For anything that goes beyond an overnight break, I have medical supervision.
If you hesitate at several points, that is not a failure. It is a precise piece of information about the right order.
And for context, because this article lists a lot of signals: most of these signs are harmless and disappear as soon as you eat more regularly again. They are not a reason to panic, they are information.
This is not about an eating window. It is about the question of whether your body currently has enough safety to occupy itself with itself instead of defending itself. That is the real reason why we are talking about fasting.
Frequently asked questions about fasting with Hashimoto's
Am I allowed to fast at all with Hashimoto's?
There is no blanket yes and no blanket no. Three things decide it: your stress level, your nutrient supply and the stability of your thyroid values.
If you are well adjusted, eat enough, sleep steadily and have normal ferritin and vitamin D values, you will usually get along with mild forms such as an extended overnight break from eating. Even that is not free of complaints for everyone. Headaches, trouble concentrating, feeling cold or disturbed sleep can occur even with a short break. If that happens, it is a signal to stop, not a sign of missing discipline. If you are exhausted, sleep badly, are underweight or have only just been newly adjusted, you should not try longer fasting without medical supervision.
When trying to conceive, during pregnancy and while breastfeeding, fasting is not an option. There the risk clearly outweighs everything else.
Why does T3 fall during fasting?
The body saves energy as soon as less food arrives. It slows the conversion of T4 into the active T3 and shifts more T4 into the inactive reverse T3. In the specialist literature this pattern is called low T3 syndrome or nonthyroidal illness syndrome.
In one trial on alternate day fasting, T3 was lower in people who had practised this form over a longer period. That was a cross-sectional comparison within the study, not a randomised arm. No cause and effect conclusion can be drawn from it. In a controlled study using the 16:8 rhythm over eight weeks, T3 fell as well, while TSH stayed unchanged.
Important: this is first of all an adaptation, not a disease. With Hashimoto's, though, you have less reserve, because your glandular tissue is already under strain. That is why you feel the same adaptation earlier.
Can fasting lower thyroid antibodies?
There are no robust human data on this so far. There is no good study that has measured TPO antibodies under fasting in Hashimoto's.
What does exist are animal data on a fasting mimicking diet in the mouse model of multiple sclerosis, where more regulatory T cells and fewer inflammation promoting cytokines were observed. And there are controlled human data on fasting followed by a vegetarian diet in rheumatoid arthritis, where a meaningful long term effect was found.
Both make an inflammation dampening effect biologically plausible. It does not prove that antibodies go down in Hashimoto's. I consider that a reasoned hypothesis, not established knowledge.
When do I take my levothyroxine if I have an eating window?
Levothyroxine is a prescription-only thyroid hormone. It is dosed individually by a doctor and monitored through blood values, because an adjustment set too high can over time favour cardiac arrhythmias and put strain on bone density, while one set too low brings the symptoms back.
It is best absorbed on an empty stomach, at a distance from coffee, dairy products, calcium and iron preparations. Acid blockers, colestyramine, sevelamer and antacids can also interfere with absorption. A fasting window can make this fasted phase easier. The risk lies elsewhere: in the shifted routine.
Data from Ramadan show how sensitively the adjustment reacts to shifted intake times: in an analysis of 481 treated people the median TSH rose from 2.0 to 2.9 µIU/mL, and 25.5 percent of those previously well adjusted were no longer in the target range afterwards.
A meta-analysis of 14 studies also found that no intake time was superior to the others. So choose the time you can keep to reliably. Do not change it on your own. Discuss it medically and have your values rechecked.
What is better with Hashimoto's: 16:8 or multi-day fasting?
From the point of view of the thyroid axis, short breaks are the gentler path. The longer the period without food, the more strongly T3 falls and the more clearly cortisol rises.
A meta-analysis across 13 studies with 357 participants showed a very strong rise in cortisol under actual fasting, while very low calorie and moderately calorie reduced diets did not trigger a significant rise. The dose makes the difference here.
Multi-day fasting with an autoimmune condition therefore belongs in medical supervision, not in solo experiments. A calm overnight break from eating is the more sensible entry point for most people.
I am a woman and I feel worse when fasting than my partner does. Am I imagining it?
No. The female cycle is closely tied to energy availability. The brain may read a lack of food as a signal to wind down reproduction. The Endocrine Society describes this connection in its guideline on functional hypothalamic amenorrhoea. Cycle shifts and absent ovulation can appear earlier than any lab value becomes conspicuous.
On top of that, Hashimoto's affects women far more often than men, and iron deficiency is common in menstruating women. Anyone who fasts with empty stores feels the energy withdrawal earlier and more strongly.
By the way: many popular fasting protocols rest on studies in men. The well known 16:8 investigation over eight weeks was carried out exclusively in resistance trained men. Your experience is physiology, not imagination.
Which lab values should I know before trying to fast?
A picture that goes beyond TSH is useful: free T3, free T4, TPO antibodies, plus ferritin, vitamin D, selenium, zinc, vitamin B12 and a blood count.
Ferritin matters particularly, because thyroid peroxidase is an iron containing haem enzyme and may work less well in iron deficiency. A review described that in two thirds of women with persistent symptoms despite an appropriate levothyroxine dose, the symptoms improved when ferritin was raised above 100 µg/L.
And free T3 absolutely belongs in the picture. In several investigations T3 fell while TSH barely moved. Anyone who measures only TSH does not see this shift.
Does autophagy automatically make fasting sensible in autoimmune conditions?
Autophagy is a real and fascinating mechanism, but it is hard to measure in humans and its effect on autoimmunity is not settled.
In an investigation in mice and humans, autophagy markers rose in the mouse liver under intermittent fasting, but not in muscle. In 50 women who fasted for 24 hours on three days per week over eight weeks the picture was mixed: after 24 hours of fasting one marker rose, while in the group that additionally took in less energy several markers were lower after twelve hours. The authors attribute this more to the weight loss.
So autophagy is not a switch that 16 hours without food flips reliably. It is an argument for plausibility, not evidence of a benefit in Hashimoto's.
How do I end a longer fasting phase safely?
Slowly and with support. After a longer period without food, coming back to eating can itself become a risk.
In refeeding syndrome, phosphate, potassium and magnesium shift abruptly into the cells under the rise in insulin, which can put strain on heart rhythm and nervous system. Those particularly at risk are people who are underweight, who have gone long without food or who have a history of an eating disorder.
With an extended overnight break this plays no role. The longer the fast, the more cautious and the more medically supervised the return to eating should be.
Can I do something else for the inflammation instead of fasting?
Yes, and for many people with Hashimoto's that is the smarter first step. Sleep, blood sugar stability, enough protein, a filled iron store, a good vitamin D status and a settled selenium supply are the foundation.
For selenium, a meta-analysis of 35 studies suggests that TPO antibodies may fall under selenium supplementation, with a standardised mean difference of minus 0.96. The certainty of this evidence was rated only as moderate, and a Cochrane review came to a more reserved verdict in 2013 on a narrower data basis. Selenium is not a substitute for thyroid hormone, it has a narrow safety range and it belongs measured before it is taken.
The decisive difference: in my view these basics cost the thyroid axis less energy than a fasting stimulus costs it. That is why the foundation comes first.
Is fasting with Hashimoto's possible during pregnancy or when trying to conceive?
No. When trying to conceive, during pregnancy and while breastfeeding, fasting is not a sensible tool. The thyroid has a markedly higher demand during this time, and the development of the child depends directly on the mother's hormone supply.
A systematic review of 43 studies on Ramadan fasting in pregnancy found no strong evidence of harm overall, but an overall weak data situation. Only three works reached high quality, and one of them found a lower birth weight. Individual studies of medium quality also found unfavourable effects on fetal growth.
With an existing autoimmune condition of the thyroid, this is not a field for experiments.
How do I notice that fasting is not doing me good?
Watch the quiet signals: feeling cold in a way you did not before. Hair loss that sets in after a few weeks. A resting pulse that becomes strikingly low or strikingly high. Changes in your cycle. Sleep that breaks off between three and four in the morning. And an exhaustion that does not improve after eating.
These are typical signs that your body is saving energy rather than tidying up. Then less fasting is the better path, not more discipline.
And one clear limit: with a racing or stumbling heartbeat, fainting, severe dizziness or confusion, stop fasting immediately and get medical help. In an emergency call 112, not an appointment for next week.
And one more thing: if fasting mainly makes you feel proud and hardly ever relaxed, that is a signal too. Fasting should make you calmer, not more tense.
Where this topic connects
Fasting with Hashimoto's never stands alone. It touches hormones, gut, stress physiology and nutrient supply at the same time.
Guide Fasting
All articles on forms of fasting, metabolism and safety.
Current sectionHormone disruption and environment
Why hormone axes fall out of rhythm and which environmental factors play a part.
HormonesThyroid and nutrients
Selenium, iron, iodine and vitamin D in their interplay with the thyroid.
GuideStress, sleep and exhaustion
Why the HPA axis co-determines almost every other system.
GuideSources
All studies were cross-checked via PubMed and are linked with a DOI. On the specific question of how fasting affects Hashimoto's thyroiditis, no controlled intervention studies exist so far. The connections presented here therefore rest on physiology, on fasting studies in people without thyroid disease, on data from other autoimmune conditions and on animal models. That is biologically plausible, but not documented with the same certainty as it would be through large randomised trials in the target group.
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This text does not replace medical advice or individual diagnostics. Changes to an existing thyroid therapy always belong in medical hands.