The supplemental form of iodine, an element required to make thyroid hormone and obtainable from iodised salt, dairy, seafood and seaweed. It is also the compound used in high-dose thyroid-blocking tablets after radiation incidents, which is an entirely different use.
Iodine is the textbook case of a nutrient where both deficiency and excess cause the same organ to malfunction. Deficiency in pregnancy is the world's leading avoidable cause of impaired brain development in children, and universal salt iodisation is one of the most successful public health interventions ever undertaken. Excess causes thyroid dysfunction too — it can induce hypothyroidism through the Wolff-Chaikoff effect or hyperthyroidism in people with nodular goitre, and doses in some seaweed and "thyroid support" products reach thousands of micrograms against an RDA of 150. Iodine deficiency has re-emerged in some high-income countries as people move away from iodised salt and dairy, so the answer is neither ignoring it nor megadosing it.
Regulation: The RDA is 150 µg/day for adults, 220 µg in pregnancy and 290 µg in breastfeeding, with a Tolerable Upper Intake Level of 1,100 µg/day. High-dose potassium iodide for radiation emergencies is a regulated medicine at 130 mg — nearly a thousand times the RDA — and is not a supplement.
What the research used
The amounts, forms and durations published trials of Iodine (potassium iodide) actually administered.
These are the amounts published trials administered — not a recommendation, and not a dose to copy. Trial participants were screened, supervised and given a characterised preparation. What is in a retail bottle is frequently not what was studied, and the amount that suits one person is not general advice.
Studied for Recommended Dietary Allowance, adults
150–290 µg
Strong
150 µg/day for adults, 220 µg in pregnancy and 290 µg while breastfeeding. Professional bodies in the US recommend 150 µg/day of supplemental iodine for women who are pregnant, breastfeeding or planning pregnancy.
Studied for Tolerable Upper Intake Level, adults
1100 µg
Strong
Set on thyroid dysfunction from excess. People with existing thyroid disease can react adversely well below this figure, in either direction.
Studied for Typical supplement or prenatal
150–225 µg
Moderate
Potassium iodide tablet
Surveys of prenatal products have found many contain no iodine or amounts differing considerably from the label — a meaningful problem given that pregnancy is when it matters most.
This is general information about food, not medical advice. It cannot diagnose, treat or replace guidance from a clinician who knows your history.
When it was taken
Each window says whether a trial compared it against an alternative, or whether it is convention and pharmacology. Most timing advice is the second kind.
None of the timings below were directly compared in a trial. They reflect how the research happened to dose, or what the pharmacology implies — which is not the same as a timing being better.
🕐Any time
Limited
Iodine is absorbed efficiently in any form and thyroid stores turn over slowly, so time of day has no established relevance.
Timing was not directly compared. This reflects convention or how the compound behaves in the body, not a trial showing this window works better.
The evidence in depth
Best supported: Not enough
What each association actually rests on, and the papers behind it. A rating describes the state of the research, not how promising the supplement sounds. How we rate evidence.
Iodine supplements are widely sold for fatigue and metabolism on the reasoning that the thyroid governs metabolic rate. In people who are not iodine-deficient, supplementation does not improve thyroid function or energy, and excess can push the thyroid in either direction.
Not enough evidence. Not enough human research to say anything useful yet.
Iodine deficiency in pregnancy impairs fetal brain development, and correcting it in deficient populations produces clear improvements in child neurological outcomes — the evidence from severely deficient regions is among the strongest in nutrition. In iodine-sufficient populations, supplementation trials have not shown a cognitive benefit, and one well-conducted trial found no effect on child IQ.
What may be going on: Thyroid hormone is built around iodine atoms and directs fetal neuronal migration and myelination; the fetus depends on maternal supply through the first trimester.
Not enough evidence. Not enough human research to say anything useful yet.
No source is attached to this association yet, so it is rated not enough evidenceby default. It is listed because people commonly ask about it — not because there is evidence for it.
Things to know
Interactions, contraindications and dose limits, most serious first. Concentrated extracts interact with medication in ways the whole plant does not, so this section carries more weight here than it does on a food page.
Too much
Both too little and too much cause thyroid disease
Excess iodine can induce hypothyroidism by shutting down hormone synthesis, or hyperthyroidism in people with nodular goitre or latent autoimmune thyroid disease. Population data show a U-shaped relationship between iodine intake and thyroid dysfunction. More is emphatically not better here.
Worth knowing
Existing thyroid disease
People with Hashimoto thyroiditis, Graves disease, nodular goitre or a history of thyroid surgery or radioiodine can react to supplemental iodine at doses well below the upper limit. Any iodine supplement in that situation is a conversation with the treating clinician, not a self-directed purchase.
Iodine can alter the effect of antithyroid drugs and levothyroxine. Combined with potassium-sparing diuretics or ACE inhibitors, potassium iodide adds a small potassium load. Amiodarone and iodinated contrast media are large hidden iodine sources.
Allergy
Iodine sensitivity and high-dose reactions
High doses can cause a metallic taste, burning mouth, gastrointestinal upset and, in sensitive people, rashes or salivary gland swelling. Reactions to iodinated contrast media are a separate phenomenon from dietary iodine.
In the food catalogue
The whole-food form, where one exists. The evidence for a concentrated extract rarely transfers to the food, or the other way round — these are cross-links, not equivalents.