Iodine
In moderately iodine-deficient schoolchildren, iodine improved 4 of 7 cognitive tests. In mildly deficient pregnant women, supplementation did not change child IQ or development in pooled trials. Cochrane found too little data for firm conclusions in pregnancy. In a Chinese cohort, above-adequate and excessive iodine intake went with more subclinical hypothyroidism and autoimmune thyroiditis.
Add to Bio HackChecked 2026-11-09
- Status
- Dietary supplement in the US. Potassium iodide is also an FDA-approved over-the-counter drug for radiation emergencies.
- Also called
- potassium iodide, KI, iodized salt, kelp, seaweed supplements, Lugol's solution, iodine drops, prenatal iodine
- Taken for
- thyroid health, pregnancy, child brain development, IQ, metabolism, energy, weight loss, goiter
Strong several reviews agree · Moderate one review or consistent trials · Limited one or a few small studies · Untested no study in people, listed with what it rests on
Shown to help2
- LimitedLimited evidence: iodine slightly raised cognitive scores in iodine-deficient children, with each result resting on one trial.
- LimitedLimited evidence: in single trials, iodine lowered hypothyroidism in children and deaths in infants. Effects on goiter were very uncertain, and weight did not change.
Reported risks4
- LimitedLimited evidence: higher iodine intake was linked to more subclinical hypothyroidism, 2.6% to 2.9% against 0.2%, in a 5-year study of Chinese regions.Common
- LimitedLimited evidence: thyroid nodules were linked to iodine deficiency, not high intake. High urinary iodine was linked to thyroid cancer, while dietary intake was not.How often is not known
- LimitedOne small trial found more digestive intolerance on iodine in pregnant women.How often is not known
- LimitedLimited evidence: 7 of 23 Swiss prenatal multivitamins contained no iodine, based on their listed contents.How often is not known
What people take it for, and what was found
Shown to help2
Cognition in iodine-deficient children
Limited evidence: iodine slightly raised cognitive scores in iodine-deficient children, with each result resting on one trial.
The numbers and sources
- Effect
- After 24 weeks, iodine improved 4 of 7 cognitive and motor tests against placebo, including rapid target marking, symbol search, rapid object naming and Raven's Coloured Progressive Matrices. Total thyroxine rose about 40% and low thyroxine fell below 1% [1]. A 2026 Cochrane review of 8 RCTs (2,528 children and adolescents) found a slight rise in cognitive function against placebo (SMD 0.19, 95% CI 0.04 to 0.34), from 1 study of 166 participants rated high certainty [2].
- Population
- 310 children aged 10 to 12 in rural Albania, median urinary iodine 43 µg/L at baseline, 87% with goiter [1]. Trials in seven countries from 1982 to 2009 [2].
- Sources
- [1], [2]
- Caveats
- Each cognition result rests on one trial in children with deficiency. The Albanian trial used a single 400 mg dose of iodized oil [1]. The Cochrane trials were old and mostly gave a single large dose [2].
Goiter, hypothyroidism and growth in children
Limited evidence: in single trials, iodine lowered hypothyroidism in children and deaths in infants. Effects on goiter were very uncertain, and weight did not change.
The numbers and sources
- Effect
- In the Cochrane review, iodine likely lowered hypothyroidism (RR 0.03, 95% CI 0.00 to 0.23, 1 study, 264 participants) and all-cause death in infants (RR 0.32, 95% CI 0.14 to 0.73, 1 study, 617 infants). Weight did not differ (MD -0.20 kg, 95% CI -1.92 to 1.52, 3 studies). The effect on goiter was very uncertain (RR 0.68, 95% CI 0.15 to 3.07, 2 studies, I² 97%), and removing the study at high risk of bias gave RR 0.31 (1 study, 230 participants) [2].
- Population
- Children and adolescents in 8 RCTs. One study gave 150 µg a day for 28 weeks, one gave infants under 1 year a single 100 mg dose, and the rest gave 400 to 490 mg once to children aged 6 to 17 [2].
- Sources
- [2]
- Caveats
- Each outcome rests on one to three trials. No study reported adverse events related to iodine [2].
Mixed results1
Thyroid function in pregnancy
Mixed results: reviews agree iodine lowered maternal thyroglobulin in pregnancy, but disagree on whether TSH changed.
The numbers and sources
- Effect
- Most studies found no effect on maternal or infant TSH or free thyroxine. Most RCTs found lower maternal thyroglobulin, and 3 found less growth in thyroid volume [3]. Cochrane found no clear difference in hypothyroidism in pregnancy or postpartum and postpartum hyperthyroidism fell (RR 0.32, 95% CI 0.11 to 0.91, 3 trials, 543 women) [5]. A 2026 meta-analysis found lower TSH (SMD -0.14, 95% CI -0.23 to -0.05) and lower thyroglobulin (SMD -0.22, 95% CI -0.32 to -0.13) with supplementation, with larger TSH drops at 200 to 300 µg a day [6].
- Population
- 37 publications in mildly to moderately deficient women [3]; 14 trials, 11 contributing data from over 2,700 women [5]; iodine-deficient pregnant women in English and Chinese studies [6].
- Sources
- [3], [5], [6]
- Caveats
- Cochrane rated the evidence low or very low quality and found the data insufficient for meaningful conclusions [5]. The reviews agree on lower thyroglobulin and differ on TSH [3][6]. The 2026 abstract does not give the number of studies or women [6].
Not shown to help1
Child development after iodine in pregnancy
Not shown to change child cognition, language or motor scores after iodine in pregnancy in women with mild to moderate deficiency.
The numbers and sources
- Effect
- Pooled trials found no effect on child cognitive scores (mean difference -0.18, 95% CI -1.22 to 0.87), language or motor scores [3]. In the largest trial, verbal IQ at age 5 to 6 was 89.5 on 200 µg a day and 90.2 on placebo (difference -0.7, 95% CI -2.9 to 1.5), and performance IQ 97.5 against 99.1 [4].
- Population
- Mildly to moderately iodine-deficient pregnant women, 10 RCTs in the review with 2 pooled for cognition [3]; 832 women in India and Thailand, 313 children analyzed [4].
- Sources
- [3], [4]
- Caveats
- Only 1 of 3 trials on child development was adequately powered [3]. These results do not describe severe deficiency. The trial was funded in part by the Nestlé Foundation [4].
Claimed, not studied in people1
Metabolism, energy and weight loss
Not studied in people. Iodine is needed to make thyroid hormone, but in mice low iodine shrank fat stores, and in people high intakes went with more underactive thyroid.
No study in people. This rests on how it acts in the body, animal studies.
The numbers and sources
- Effect
- Iodine is an essential part of the thyroid hormones thyroxine and triiodothyronine, and both too little and too much can cause hypothyroidism or hyperthyroidism [13]. In mice kept on a low-iodine diet, T4 fell and TSH rose, fat made up about 2.3 times more of the energy burned, and fat depots were about 60% smaller than on a normal diet, while oxygen use and activity did not change [14]. In a Chinese cohort, subclinical hypothyroidism developed more often in regions with more than adequate or excessive iodine intake than where intake was mildly deficient [7].
- Population
- A review [13]. C57BL/6 male mice [14]. 3,018 people in three regions of China [7].
- Sources
- [13], [14], [7]
- Caveats
- No trial measured weight, energy or metabolic rate in people given iodine. The mouse result runs opposite to the weight loss claim and does not show what supplements do in people [14].
Doses studied
The pregnancy trial used 200 µg of iodine a day until delivery [4]. The Albanian trial gave one oral dose of 400 mg as iodized oil [1]. Trials in the children's Cochrane review gave 150 µg a day for 28 weeks, or a single dose of 100 mg to infants and 400 to 490 mg to older children [2]. In a Chinese cohort, median urinary iodine was 84, 243 and 651 µg/L in regions of mildly deficient, more than adequate and excessive intake [7]. EFSA sets a tolerable upper intake level of 600 µg a day for adults, including during pregnancy and lactation [8].
Risks
Reported risks3
Thyroid disease at high intake
Limited evidence: higher iodine intake was linked to more subclinical hypothyroidism, 2.6% to 2.9% against 0.2%, in a 5-year study of Chinese regions.
The numbers and sources
- How common
- Common: between 1 in 100 and 1 in 10 people.
- Effect
- Over 5 years, subclinical hypothyroidism developed in 0.2%, 2.6% and 2.9% of people with mildly deficient, more than adequate and excessive iodine intake. Autoimmune thyroiditis developed in 0.2%, 1.0% and 1.3%. People with thyroid antibodies at baseline had more raised TSH at the higher intakes [7].
- Population
- 3,018 people in three regions of China, followed 1999 to 2004 [7].
- Sources
- [7]
- Caveats
- Observational comparison of regions, not a trial of supplements.
Digestive intolerance in pregnancy
One small trial found more digestive intolerance on iodine in pregnant women.
The numbers and sources
Prenatal vitamins with no iodine
Limited evidence: 7 of 23 Swiss prenatal multivitamins contained no iodine, based on their listed contents.
The numbers and sources
- How common
- How often is not known: the sources give no rate.
- Effect
- Of 23 prenatal multivitamins on the Swiss market, 7 (30.4%) contained no iodine. Of 44 multivitamins overall, 18 (40.9%) contained 150 µg or more [12].
- Population
- Products, not people [12].
- Sources
- [12]
- Caveats
- The survey compiled product contents and did not test whether they matched the label [12].
Mixed results1
Thyroid nodules and thyroid cancer
Limited evidence: thyroid nodules were linked to iodine deficiency, not high intake. High urinary iodine was linked to thyroid cancer, while dietary intake was not.
The numbers and sources
- How common
- How often is not known: the sources give no rate.
- Effect
- In 25 cross-sectional studies (54,621 people), thyroid nodules were more common with iodine deficiency than adequate status (OR 1.28, 95% CI 1.09 to 1.50), and did not differ with more than adequate (OR 1.02) or excessive intake (OR 1.13, 95% CI 0.98 to 1.30) [9]. A second review of 31 studies found the same pattern, with OR 1.24 for deficiency and no significant link for higher intake [10]. In 17 studies (273,651 people), high urinary iodine was linked to thyroid cancer (OR 6.43, 95% CI 2.72 to 15.22), while dietary iodine intake was not (OR 0.75, 95% CI 0.37 to 1.52) [11].
- Population
- Population and clinical studies of iodine status [9][10][11].
- Sources
- [9], [10], [11]
- Caveats
- Observational studies, mostly cross-sectional for nodules, which cannot show cause [9][10]. Urinary iodine is a single measure of recent intake. None of these studies tested supplements.
Claims tested and not found
- That iodine in mildly deficient pregnancies raises child IQ. Verbal and performance IQ did not differ from placebo at age 5 to 6 [4].
Sources
- Zimmermann MB, Connolly K, Bozo M, et al. Iodine supplementation improves cognition in iodine-deficient schoolchildren in Albania: a randomized, controlled, double-blind study. The American Journal of Clinical Nutrition. 2006. https://doi.org/10.1093/ajcn/83.1.108
- Velasco I, Rueda-Etxebarria M, Trak-Fellermeier MA, et al. Iodine supplementation for preventing iodine deficiency disorders in children and adolescents. Cochrane Database of Systematic Reviews. 2026. https://doi.org/10.1002/14651858.CD014475.pub2
- Dineva M, Fishpool H, Rayman MP, et al. Systematic review and meta-analysis of the effects of iodine supplementation on thyroid function and child neurodevelopment in mildly-to-moderately iodine-deficient pregnant women. The American Journal of Clinical Nutrition. 2020. https://doi.org/10.1093/ajcn/nqaa071
- Gowachirapant S, Jaiswal N, Melse-Boonstra A, et al. Effect of iodine supplementation in pregnant women on child neurodevelopment: a randomised, double-blind, placebo-controlled trial. The Lancet Diabetes & Endocrinology. 2017. https://doi.org/10.1016/S2213-8587(17)30332-7
- Harding KB, Peña-Rosas JP, Webster AC, et al. Iodine supplementation for women during the preconception, pregnancy and postpartum period. Cochrane Database of Systematic Reviews. 2017. https://doi.org/10.1002/14651858.CD011761.pub2
- Lv Y, Jin B, Guo J, et al. Effects of different iodine supplementation strategies on thyroid function in iodine-deficient pregnant women: a meta-analysis. Frontiers in Public Health. 2026. https://doi.org/10.3389/fpubh.2026.1846475
- Teng W, Shan Z, Teng X, et al. Effect of Iodine Intake on Thyroid Diseases in China. New England Journal of Medicine. 2006. https://doi.org/10.1056/NEJMoa054022
- European Food Safety Authority. Overview on Tolerable Upper Intake Levels as derived by the Scientific Committee on Food (SCF) and the EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA). Version 11. 2025. https://www.efsa.europa.eu/sites/default/files/assets/UL_Summary_tables.pdf
- Lu C, Dong H, Shi P, et al. Correlations between iodine status and the risk of thyroid nodules, a systematic review and dose-response meta-analysis. Frontiers in Endocrinology. 2026. https://doi.org/10.3389/fendo.2026.1711749
- Gräfe W, Scheibe S, Schwarz J, et al. The myth of iodine: A systematic review and meta-analysis on the relationship between iodine and thyroid nodule. Journal of Endocrinological Investigation. 2025. https://doi.org/10.1007/s40618-025-02606-4
- Li S, Yu H. Dietary and Urinary Iodine in Relation to Thyroid Cancer Risk: A Meta-Analysis. Cancer Control. 2026. https://doi.org/10.1177/10732748261419192
- Gfeller M, Colque G, Kopp PA. Iodine content of frequently used prenatal and adult multivitamins in Switzerland. Frontiers in Endocrinology. 2022. https://doi.org/10.3389/fendo.2022.1041232
- Moleti M, Di Mauro M, Paola G, et al. Nutritional iodine status and obesity. Thyroid Research. 2021. https://doi.org/10.1186/s13044-021-00116-y
- Bocco BMLC, Fernandes GW, Fonseca TL, Bianco AC. Iodine deficiency increases fat contribution to energy expenditure in male mice. Endocrinology. 2020. https://doi.org/10.1210/endocr/bqaa192