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Thyroid Health

Iodine

Evidence Level
Very Strong
1 Clinical Trial
4 Documented Benefits
5/5 Evidence Score

Iodine is an essential trace mineral that the thyroid uses to make thyroid hormones, which help regulate metabolism and are needed for normal brain and bone development before and after birth. Most people in the United States get enough from iodized salt, dairy, seafood and eggs. Needs rise in pregnancy and breastfeeding, and the American Thyroid Association advises a 150 mcg supplement then. In mildly deficient children a 150 mcg supplement improved some thinking skills, but trials in mildly deficient pregnant women have not shown better child development. Too little and too much iodine can both disrupt the thyroid, and some iodine-only supplements exceed the 1,100 mcg daily upper limit, so people with thyroid conditions should talk to their doctor before supplementing.

Studied Dose RDA 150 mcg a day for adults, 220 mcg in pregnancy and 290 mcg when breastfeeding; upper limit 1,100 mcg a day. Trials cited here used 150 to 200 mcg a day.
Active Compound Iodide, usually as potassium iodide or sodium iodide; some supplements use kelp.
Deficiency information View details

Iodine deficiency was once common in mountainous areas of the United States and in the goiter belt around the Great Lakes; iodized salt, sold in the US since the 1920s, and a wider food supply have made overt deficiency uncommon in North America. Worldwide it remains the most common cause of preventable intellectual disability. Surveys suggest many pregnant women in the US may get too little iodine, and US iodine levels fell by more than half between the early 1970s and early 1990s before stabilizing.

Common symptoms

  • Enlarged thyroid (goiter), usually the earliest clinical sign
  • Hypothyroidism when intake falls very low (below about 10 to 20 mcg a day)
  • Impaired mental function and work productivity linked to hypothyroidism
  • In children, somewhat lower than average IQ
  • In severe deficiency during pregnancy: miscarriage, stillbirth, and cretinism in the child, which is irreversible

At-risk groups

  • Pregnant and breastfeeding women (RDA 220 and 290 mcg a day; the American Thyroid Association advises a 150 mcg potassium iodide supplement)
  • Women planning a pregnancy (also covered by the American Thyroid Association advice)
  • People who do not use iodized salt (sea salt, kosher salt and Himalayan salt are not usually iodized)
  • Vegans and people who eat little or no dairy, seafood or eggs
  • People in regions with iodine-poor soils who eat mostly local food
  • People with marginal iodine status who eat a lot of goitrogenic foods such as soy, cassava and cruciferous vegetables
When to see a doctor: Visible swelling at the front of the neck, or symptoms you think may be thyroid related, are worth discussing with a doctor, who can order thyroid blood tests. Anyone planning a pregnancy or newly pregnant should check that their prenatal supplement contains iodine, because some do not. Too much iodine, including from high-dose supplements, can also disrupt thyroid function.
Explore related deficiencies →
Sources: NIH ODS: Iodine Fact Sheet for Health Professionals · Teratology Society position paper: iodine deficiency in pregnancy (PMC3637994)

Benefits

Thyroid hormone production

Iodine is an essential part of the thyroid hormones thyroxine (T4) and triiodothyronine (T3), which help regulate metabolism. In mildly iodine-deficient children, 150 mcg a day brought blood thyroglobulin, a marker of iodine status, down to 8.5 mcg/L versus 11.6 on placebo, and in mildly to moderately deficient pregnant women supplements lowered thyroglobulin and in some trials limited thyroid enlargement.

Supported by Trial 1

Thyroid hormone and metabolic rate

Thyroid hormones are critical determinants of metabolic activity, and iodine is needed to make them. Very low intakes, below about 10 to 20 mcg a day, lead to hypothyroidism, which is often accompanied by goiter. Surveys show the general US population gets enough iodine, so extra iodine matters mainly for people whose intake is low.

Supported by Trial 1

Fetal and infant brain development

Thyroid hormones are needed for normal brain and skeletal development in fetuses and infants, and chronic, severe iodine deficiency in pregnancy causes cretinism. In a mildly deficient UK cohort, children of mothers with low iodine in early pregnancy were more likely to have low verbal IQ and reading scores, but pooled trials of iodine supplements in mildly to moderately deficient pregnant women have not shown better child cognition.

Supported by Trial 1

Thinking skills in mildly iodine-deficient children

In a randomized, placebo-controlled trial in 184 mildly iodine-deficient New Zealand children aged 10 to 13, 150 mcg a day for 28 weeks improved 2 of 4 cognitive subtests, both measures of perceptual reasoning, and raised the overall cognitive score by 0.19 standard deviations. The other two subtests did not change.

Supported by Trial 1

Mechanism of action

1

Thyroid hormone synthesis

Iodide is actively transported into thyroid follicular cells by the sodium-iodide symporter, oxidized by thyroid peroxidase (TPO), and incorporated into tyrosine residues on thyroglobulin to form T3 and T4.

2

TSH control of the thyroid

Thyroid hormone output is controlled by thyroid-stimulating hormone (TSH) from the pituitary. When iodine is short, TSH stays elevated and the thyroid enlarges (goiter) as it tries to trap more iodine.

Clinical trials

1
Iodine and cognition in mildly deficient children (RCT)
PubMed

Randomized, double-blind, placebo-controlled trial of a daily tablet with 150 mcg iodine or placebo for 28 weeks, with 4 subtests of the Wechsler Intelligence Scale for Children. (Gordon et al. 2009, Am J Clin Nutr)

184 children aged 10 to 13 in Dunedin, New Zealand, mildly iodine deficient at baseline (median urinary iodine 63 mcg/L).

Iodine status improved (median urinary iodine 145 mcg/L versus 81 on placebo). Picture concepts and matrix reasoning scores improved, letter-number sequencing and symbol search did not, and the overall cognitive score was 0.19 standard deviations higher than placebo (p = 0.011).

Side effects and drug interactions

Common Potential side effects

Too much iodine can cause goiter, raised TSH and hypothyroidism, and can also cause hyperthyroidism or thyroiditis; people with autoimmune thyroid disease or past deficiency can react at intakes that are safe for others.
Acute iodine poisoning is rare and usually takes doses of many grams; symptoms include burning of the mouth, throat and stomach, nausea, vomiting and diarrhea.
Long-term intake above the adult upper limit of 1,100 mcg a day raises the risk of adverse effects, and some iodine-only supplements exceed it.

Important Drug interactions

Antithyroid medicines such as methimazole: high doses of iodine can add to their effect and could cause hypothyroidism.
ACE inhibitors and potassium-sparing diuretics such as spironolactone: taking potassium iodide with them can raise the risk of high blood potassium (hyperkalemia).

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Frequently asked questions about Iodine

How much iodine should I take?

The RDA is 150 mcg per day for adults, and 220 to 290 mcg in pregnancy and breastfeeding. Most people in countries using iodized salt get enough. Supplements often provide around 150 mcg; avoid routinely taking very high doses.

What is iodine good for?

Iodine is essential for making thyroid hormones, which regulate metabolism, growth, and development. Adequate iodine is especially important during pregnancy for the baby's brain development.

Can you take too much iodine?

Yes. Both too little and too much iodine can disrupt thyroid function. Very high doses, such as from some iodine-only supplements, can trigger or worsen thyroid problems in susceptible people. Stay near the recommended amount unless a doctor advises otherwise.

Should I take iodine or kelp for my thyroid?

If you use iodized salt or eat dairy, seafood and eggs, you probably get enough already; the supplement research is in people who were short of iodine. Kelp and seaweed supplements vary widely in iodine content, from a few micrograms to several thousand per dose. Check with your doctor before taking iodine for a thyroid concern.

What is Iodine?

Iodine is an essential trace mineral that the thyroid uses to make thyroid hormones, which help regulate metabolism and are needed for normal brain and bone development before and after birth. Most people in the United States get enough from iodized salt, dairy, seafood and eggs.

What is Iodine used for?

Iodine is researched primarily for Thyroid Health. Iodine is an essential part of the thyroid hormones thyroxine (T4) and triiodothyronine (T3), which help regulate metabolism.

What are the signs of Iodine deficiency?

Iodine deficiency was once common in mountainous areas of the United States and in the goiter belt around the Great Lakes; iodized salt, sold in the US since the 1920s, and a wider food supply have made overt deficiency uncommon in North America.

Is Iodine safe, and does it have side effects?

Side effects and cautions reported for Iodine include: Too much iodine can cause goiter, raised TSH and hypothyroidism, and can also cause hyperthyroidism or thyroiditis; people with autoimmune thyroid disease or past deficiency can react at intakes that are safe for others. It may also interact with some medications. Iodine is not right for everyone, so check with a healthcare provider first if you are pregnant or breastfeeding, have a medical condition, or take prescription medication.

Does Iodine interact with any medications?

Possible interactions include: Antithyroid medicines such as methimazole: high doses of iodine can add to their effect and could cause hypothyroidism. ACE inhibitors and potassium-sparing diuretics such as spironolactone: taking potassium iodide with them can raise the risk of high blood potassium (hyperkalemi… If you take prescription medication, check with a pharmacist or doctor before using it.

How strong is the scientific evidence for Iodine?

NutraSmarts rates the evidence for Iodine as Very Strong (5 out of 5). It is backed by 1 study summary and 8 cited references summarized on this page. A higher rating reflects more, larger, and better-designed human studies.

References(8 citations)

Evidence ratings on NutraSmarts are based on the totality of human clinical research, with emphasis on randomized controlled trials, meta-analyses, and systematic reviews. The references below directly support claims made throughout this page.

  1. Zimmermann MB, Jooste PL, Pandav CS. Iodine-deficiency disorders. Lancet. 2008;372(9645):1251-62. doi: 10.1016/S0140-6736(08)61005-3.PubMedUsed to support: Lancet seminar on iodine-deficiency disorders: about 2 billion people have insufficient intake; deficiency is the most common cause of preventable mental impairment worldwide; salt iodization is the best control strategy in nearly all countries.
  2. Zimmermann MB. Iodine deficiency. Endocr Rev. 2009;30(4):376-408. doi: 10.1210/er.2009-0011.PubMedUsed to support: Review of iodine deficiency across the life cycle: about 2 billion people have insufficient intake, about half of Europe is mildly deficient, and intakes in the US and Australia have fallen in recent years; salt iodization is the main control strategy.
  3. Bath SC, Steer CD, Golding J, Emmett P, Rayman MP. Effect of inadequate iodine status in UK pregnant women on cognitive outcomes in their children: results from the Avon Longitudinal Study of Parents and Children (ALSPAC). Lancet. 2013;382(9889):331-7. doi: 10.1016/S0140-6736(13)60436-5.PubMedUsed to support: Cohort of 1,040 UK women with first-trimester urine samples: children of mothers with an iodine-to-creatinine ratio below 150 mcg/g were more likely to score in the lowest quartile for verbal IQ at age 8 and reading at age 9. Observational, not a supplement trial.
  4. Zhou SJ, Anderson AJ, Gibson RA, Makrides M. Effect of iodine supplementation in pregnancy on child development and other clinical outcomes: a systematic review of randomized controlled trials. Am J Clin Nutr. 2013;98(5):1241-54. doi: 10.3945/ajcn.113.065854.PubMedUsed to support: Systematic review of 8 RCTs: supplementation in severe deficiency reduced cretinism but did not improve childhood intelligence, growth or pregnancy outcomes; none of the 6 trials in mild-to-moderate deficiency reported child development; effects on thyroid function were inconsistent.
  5. Dineva M, Fishpool H, Rayman MP, Mendis J, Bath SC. 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. Am J Clin Nutr. 2020;112(2):389-412. doi: 10.1093/ajcn/nqaa071.PubMedUsed to support: Systematic review and meta-analysis (10 RCTs among 37 publications) in mildly to moderately deficient pregnant women: most trials showed no effect on maternal or infant TSH or free T4, most found lower maternal thyroglobulin, 3 limited the rise in thyroid volume, and pooled data from 2 RCTs showed no effect on child cognitive, language or motor scores.
  6. Gordon RC, Rose MC, Skeaff SA, Gray AR, Morgan KM, Ruffman T. Iodine supplementation improves cognition in mildly iodine-deficient children. Am J Clin Nutr. 2009;90(5):1264-71. doi: 10.3945/ajcn.2009.28145.PubMedUsed to support: Randomized, double-blind trial in 184 mildly iodine-deficient New Zealand children aged 10 to 13: 150 mcg iodine a day for 28 weeks improved iodine status, left thyroglobulin lower than placebo (8.5 versus 11.6 mcg/L), improved 2 of 4 cognitive subtests and raised the overall cognitive score by 0.19 SD versus placebo.
  7. Office of Dietary Supplements, National Institutes of Health. Iodine: Fact Sheet for Health Professionals. NIH Office of Dietary Supplements. 2024;Updated November 5, 2024..SourceUsed to support: Not PubMed-indexed (US government fact sheet). Gives the RDAs (150 mcg adults, 220 mcg pregnancy, 290 mcg lactation) and the 1,100 mcg adult upper limit; thyroid hormone roles and TSH control; groups at risk (no iodized salt, pregnancy, vegans and low dairy or seafood intake, iodine-poor soils, goitrogens with marginal status); the American Thyroid Association advice of a 150 mcg potassium iodide supplement before and during pregnancy and lactation; effects of excess intake; and interactions with antithyroid drugs, ACE inhibitors and potassium-sparing diuretics.
  8. Aakre I, Solli DD, Markhus MW, Mæhre HK, Dahl L, Henjum S, Alexander J, Korneliussen PA, Madsen L, Kjellevold M. Commercially available kelp and seaweed products - valuable iodine source or risk of excess intake? Food Nutr Res. 2021;65. doi: 10.29219/fnr.v65.7584.PubMedUsed to support: Analysis of 96 seaweed products on the Norwegian market: the 14 seaweed supplements supplied 5 to 5,600 mcg iodine per daily dose, and labelling was inadequate or inaccurate for several products.