Iodine and the thyroid: too little, too much, and why both cause problems

You are standing in front of the salt shelf. Two packets, almost identical. One says iodized salt, the other does not.

In the past you would simply have taken one. Since your diagnosis you read labels differently. In the forum it said that iodine is dangerous in Hashimoto. Your family doctor never mentioned it.

Many people with a thyroid condition know this moment. In my consultations I hear the question almost every week, mostly in the same form: do I have to throw out my iodized salt?

The honest answer needs a few paragraphs. Iodine is not a nutrient you can discuss with a yes or a no. It is the building material the hormone is made of. With that, the question is never whether, but how much and into which gland.

Iodine is not one nutrient among others. It is the raw material itself

Think of magnesium for a moment, or of vitamin C. Both are helpers. They stand beside things and make reactions possible.

Iodine does something else. Iodine does not sit next to the hormone, it sits inside the hormone.

The thyroid hormone T4 has the full name tetraiodothyronine. Four iodine atoms. T3 is called triiodothyronine, three iodine atoms. Roughly two thirds of the molecular weight of T4 is simply iodine. Without iodine, little hormone is not produced. None is.

The gland handles this raw material accordingly. It does not wait for iodine to come by. It actively pumps it out of the blood, against a concentration gradient.

After that, an enzyme called thyroid peroxidase, TPO for short, attaches the iodide to a large protein called thyroglobulin. This step is called organification. Remember it, it comes up again later.

The iodinated thyroglobulin is then stored in the follicle. In generous amounts. The thyroid keeps a reserve of its own raw material, depending on the starting situation for weeks to months.

The same molecule governs basal metabolic rate, heat production and the maturation of the nervous system. Seen through the KPNI lens, the thyroid sits at a crossroads of metabolism, hormonal system and nervous system. A building material for three systems is not negotiable.

And now you know why the iodine question has a different quality from the question about any given food supplement.

The deficiency side: goiter, brain development and the greatest success of this field

Look at old photographs from the Alps, from the Valais or from Tyrol. In many pictures you see thick necks.

That was no coincidence. Those were iodine deficiency areas, far from the sea, with iodine-poor soils. And what you see is the visible part of a long chain.

The chain goes like this. If too little iodine arrives, the pituitary sends more TSH. TSH is not only a production order, it is also a growth stimulus. First the gland grows evenly larger, which is called diffuse goiter. Over years nodules develop, and in some of them cells keep working without TSH. That is functional autonomy.

If you want to know how nodules and goiter are assessed today, that is covered in detail in Assessing nodules and goiter. Here I stay with iodine.

This is why iodine deficiency is still regarded as the most common preventable cause worldwide of goiter and of impairments in mental development. Michael Zimmermann puts the weighting like this in his standard review: the comparatively small risks of iodine excess are far outweighed by the substantial risks of iodine deficiency.

And now you know why a blanket recommendation to avoid iodized salt would not be a harmless one.

Where Germany stands today, and why supply is falling again

You are probably thinking: iodine deficiency, that was back then. Old photographs, a solved problem.

I told it that way for a long time too. The numbers say something else. The most reliable German data come from two surveys by the Robert Koch Institute, evaluated on behalf of the responsible federal ministry: DEGS for adults and KiGGS for children and adolescents in two waves.

32 percent of adults in DEGS did not reach the estimated average iodine requirement, and among children and adolescents it was almost 44 percent. A second data series shows the trajectory even more sharply, because it works with genuine 24-hour urine collections.

That has an uncomfortable consequence. By far the largest part of the salt you eat does not come from your shaker. It comes from bread, sausage, cheese, convenience products and eating out. If no iodized salt is used there, your shaker changes little about it.

Now back to the question from the supermarket. Does one have to avoid iodized salt?

I deliberately give you no personal recommendation on this, in either direction. What I can give you are the reference values. The European Food Safety Authority derives 150 micrograms of iodine per day for adults and 200 for pregnant and breastfeeding women. As the tolerable upper intake level it names 600 micrograms, while for Germany the lower D-A-CH value of 500 applies.

And now you know why the statement "Germany is well supplied" and the statement "supply is falling" can both be true at the same time.

Pregnancy and breastfeeding: the highest requirement in a whole lifetime

There is one phase of life in which the iodine question is no longer a matter of taste.

The requirement rises for three reasons at once. The mother produces more hormone, the kidney excretes more iodine, and part of it goes to the child, the placenta and the amniotic fluid. In the first weeks the fetus produces no hormone of its own. Exactly during this time the first major maturation phase of the nervous system takes place.

This is why the reference values are higher. EFSA derives 200 micrograms per day for pregnant and breastfeeding women, and the American professional society ATA names 250 micrograms as total intake from diet and supplements. These are numbers from guidelines, not an instruction to you.

I deliberately let this contrast stand. Documented by large cohorts and a meta-analysis is the association between maternal iodine status and child development. Not documented is that a late supplement undoes it.

There is a counter-direction here as well. A position paper of the Teratology Society points out that the iodine content in combination supplements for pregnant women varies strongly, upwards as well as downwards. Why that matters you will see in the next section: the fetus defends itself against too much iodine less well than you do.

And now you know why in this section I explicitly ask you not to decide anything on your own.

Wolff-Chaikoff and the escape phenomenon: the built-in emergency brake

Imagine a small workshop. Every day it processes a manageable amount of material into a very potent product.

Now somebody dumps a thousand times that amount in front of the door. A well-organised workshop then shuts production down temporarily. Not because it is broken, but because otherwise a thousandfold excess of product would be made.

That is exactly what your thyroid does. The reflex has had a name since 1948. Jan Wolff and Israel Chaikoff described back then that high iodide concentrations inhibit hormone synthesis instead of driving it up. When you read somewhere that too much iodine damages the thyroid, that is the echo of this work, usually without the second part.

Wolff J, Chaikoff IL. J Biol Chem. 1948;174(2):555-564. PMID: 18865621 (no DOI assigned) [Foundational study, historical]

So much for the direction in which too much iodine makes too little hormone. There is a second one, and it is the more dangerous.

Do you remember functional autonomy? Autonomous areas do not listen to TSH. They lack the regulator that brakes when supply is high, and they simply process whatever arrives. Out of this comes iodine-induced hyperthyroidism, often called Jod-Basedow. The German Federal Institute for Risk Assessment names this risk explicitly: a potentially life-threatening derailment, even after a single serving of very iodine-rich seaweed in someone with unrecognised autonomy.

Seen through the lens of toxicology this is familiar. It is not the substance that decides, but the dose, the time course and the susceptibility of the person receiving it.

And now you know why the same amount of iodine can have completely different consequences in two people.

Iodine and autoimmunity: a curve, not a direction

If you have read this far, you are waiting for an answer. Does a lot of iodine cause Hashimoto or not?

The data do not give a direction. They give a shape. And this shape is a U.

The concern about the right end of the curve comes from several countries that raised their iodine supply and then looked closely.

The same group later compared two communities with 261 versus 145 micrograms per litre. TPO antibodies were positive in 10.64 versus 8.4 percent. Both supply situations would have been called good in Germany. From northwestern Greece comes a similar finding: among 302 schoolchildren in a former deficiency region, 9.6 percent had autoimmune thyroiditis, three times as often as seven years earlier. There, however, two cross-sections were compared, and antibody tests and ultrasound had improved in the meantime.

The same working group broke down 1,682 new cases of hyperthyroidism across 2,027,208 person-years by cause. In the more iodine-poor Aalborg the incidence was 96.7 versus 60.0 per 100,000 in Copenhagen. The difference was almost entirely due to toxic nodular goiters and autonomous adenomas, while for Graves disease it was not significant. Iodine supply therefore mainly shifts which kind of hyperthyroidism arises.

And now comes the part I have not found in any German guide text on this topic.

How does that fit together? Honest answer: not completely. There is however a reading that explains a good deal.

A second review arrives at the same point. Carlotta Teti and colleagues note that it remains unclear whether the immunological changes are a direct iodine effect or a secondary response to tissue damage. They remind us that antibodies and disease are not the same thing, more on that in Lowering Hashimoto antibodies.

That leaves the question of genes. A Chinese case-control study with 1,723 people found rising odds ratios up to 2.07 across the quartiles of iodine concentration, but no significant interaction with a genetic risk score. The association therefore does not appear to be limited to genetically burdened people. The relationship there was also nearly linear rather than U-shaped. This inconsistency belongs on the table.

A systematic review from 2025 across 31 studies after universal salt iodization sums up the practical consequence: what is decisive is not whether iodization happens, but whether monitoring detects oversupply.

And now you know why honest answers to this question take longer than three sentences in a forum.

The Hashimoto question: high doses, strict avoidance, and why neither of them fits

Back to the salt shelf. You have Hashimoto and want to know what applies to you. I split this into two halves.

First half: why the caution is justified

People with autoimmune thyroiditis are on the list of those who escape the Wolff-Chaikoff effect less reliably, even when their values are currently unremarkable. That is the reason for the caution towards high-dose iodine in Hashimoto. Not a fashionable opinion, but regulatory physiology.

Farebrother and Zimmermann add: in a gland that is already damaged, an intake slightly above requirement can in some cases set off a functional disturbance. When a guideline advises against high iodine doses in autoimmune thyroiditis, it therefore has a physiological reason for it. That deserves respect, from a functional perspective too.

Second half: the study that is almost always quoted wrongly

There is exactly one randomised trial on iodine restriction in an autoimmune thyroid disease. It concerns Graves disease, and its result points in a different direction than expected.

With that the Hashimoto question can be summed up without me naming you an amount. In an autoimmune thyroid disease the target corridor is narrower than in other people. But it does not lie at zero.

From the lens of functional medicine a legitimate additional question arises here: not how much a population needs, but how high the individual total iodine load of a person currently is, with all supplements and drugs taken together. What this view cannot deliver at present is a validated marker for the individual case.

Two topics I only touch on. What nutrition can achieve in Hashimoto is covered in Nutrition in Hashimoto, and the treatment of hyperthyroidism in Graves disease and hyperthyroidism. And selenium is closely involved in iodine metabolism, because the enzymes that release iodine from the hormone need selenium. Separate article: Selenium, zinc, iron and vitamin D.

And now you know why on this matter I advise neither capsules nor strict avoidance.

The invisible iodine sources: seaweed, contrast media, amiodarone, disinfection

Now it gets practical. When somebody in Germany really does get too much iodine, it is almost never through iodized salt. Leung and Braverman put it like this: the source of the excess iodine is often not obvious.

Seaweed and kelp: practically the most important source

The same document notes at the same time that in Germany on average about a third less iodine is taken in than recommended. Both sentences stand in the same regulatory assessment. That is the double nature of this topic.

A case report from Romania shows how that can feel: a 70 year old woman with no known thyroid condition developed hyperthyroidism with a racing heart, sleeplessness and six kilograms of weight loss three months after starting a kelp-containing slimming supplement. After stopping it and receiving antithyroid treatment, her values returned to the reference range.

Iodine-containing contrast media

A Taiwanese cohort of 98,210 people arrives at the same result, only with the weighting reversed: hazard ratio 1.46 overall, 1.22 for hyperthyroidism and 2.00 for hypothyroidism. That fits a rule from Laurberg: in iodine-replete regions hypothyroidism dominates after an iodine load, in iodine-poor regions hyperthyroidism does.

Amiodarone

The European guideline distinguishes two types of amiodarone-induced thyrotoxicosis. Type 1 is an iodine-induced hyperthyroidism in a nodular gland or in latent Graves disease and is treated with thionamides. Type 2 arises from a destructive inflammation in a previously normal gland and is treated with glucocorticoids.

Disinfection and radioiodine

Povidone iodine is an excellent antiseptic, and iodine is absorbed through the skin while it is used, especially with large-area application and in newborns. Conversely Zimmermann describes how the decline of such disinfection in hospitals has made iodine supply during parenteral nutrition a topic of its own. Here too, two ends.

Radioiodine therapy uses exactly the route from above: the sodium-iodide symporter preferentially brings radioactively labelled iodine into the overactive tissue. This is why a low-iodine preparation is required before radioiodine therapy, and this is why contrast media given shortly beforehand can postpone the therapy by weeks to months. This preparation is defined and supervised by the treating nuclear medicine department, it is not something you put together yourself. Everything further in Graves disease and hyperthyroidism.

And now you know why, with unclear thyroid values, my first question is not about salt but about the shelf with the food supplements and about the list of medications.

How you can assess your iodine status, and why one urine value is not enough

If you have read this far, you probably want to measure. And this is exactly where it gets uncomfortable.

Urinary iodine concentration is an excellent marker. For populations. For you as an individual person it says surprisingly little.

That is no argument against measurement at population level, which works well. It is an argument against reading a single value as a personal diagnosis. Farebrother and colleagues discuss thyroglobulin as a supplementary marker, but for the individual case it is not established.

What carries more weight in practice is unspectacular: a thorough history of all iodine sources, meaning seaweed products, supplements, drugs and recent contrast media, plus thyroid values and ultrasound. How to read those values is covered in Thyroid blood values: which ones really count. Symptoms with unremarkable values are a topic of their own, see Normal values, symptoms anyway and Functional hypothyroidism.

And now you know why I read a single self-test for iodine in urine only as a rough pointer.

Frequently asked questions about iodine and the thyroid

Iodine, the thyroid and the rest of the body

The thyroid does not stand alone. It hangs on nutrients, on the gut barrier, on inflammation and on the burden coming from outside.

Two neighbouring topics from the existing articles connect directly: Heavy metals and Hashimoto for the toxicological side and Iron deficiency, thyroid and sleep for energy and the night. How thyroid and cycle influence each other is covered in The thyroid and female hormones.