Supplements

Iodine

Updated · 41 sources

Iodine is an essential trace element that the thyroid gland uses to build its two hormones, thyroxine (T4) and triiodothyronine (T3). Adults need very little: the US RDA is 150 mcg/day, rising to 220 mcg/day in pregnancy and 290 mcg/day during breastfeeding. Most of the US population gets enough, although some pregnant women may not.

The risk runs in both directions. Too little iodine causes goiter and, in pregnancy, can permanently harm a child’s brain development. Too much can cause hypothyroidism or hyperthyroidism in susceptible people, and a single daily dose of a kelp or seaweed product can pass the upper limit. No good evidence shows that extra iodine aids weight loss or “supports” a healthy thyroid in people who are not deficient. Separately, potassium iodide is an FDA-approved drug with one narrow emergency use: blocking radioactive iodine from entering the thyroid after a nuclear release.

This page summarizes what the NIH Office of Dietary Supplements, the FDA, drug labels, guidelines and clinical trials report. It is for education only and is not medical advice.

Property Detail
Type Essential trace element; dietary supplement ingredient; potassium iodide is also a drug
Main supplement forms Potassium iodide, sodium iodide, kelp and other seaweed
Adult RDA (US) 150 mcg/day; 220 mcg/day in pregnancy; 290 mcg/day in lactation
Upper limit US: 1,100 mcg/day for adults. EU: 600 mcg/day for adults
Typical US intake About 216 mcg/day from food (FDA Total Diet Study, 2008–2012), not counting salt added at home
How status is measured Median urinary iodine in populations; a single urine test cannot establish an individual’s status
Absorption Iodide is almost completely absorbed (96.4% for potassium iodide in a small study)
FDA-approved drug use Potassium iodide 65 mg and 130 mg tablets and 65 mg/mL solution for thyroid blocking in radiation emergencies
Half-life No single value established for a nutrient held mainly in thyroid stores; not listed here
WADA status Not named on the 2026 Prohibited List

What it is

Iodine rarely occurs as the free element. In food and supplements it appears mostly as iodide salts (such as potassium iodide), iodate and, less often, molecular iodine (I2). Soil iodine varies widely, which is why whole regions have historically been deficient and why many countries now iodize salt. US manufacturers have added iodine to table salt since the 1920s, but the practice is voluntary (NIH ODS).

Role in thyroid hormone

Iodine is a structural part of T4 and T3. These hormones regulate protein synthesis, enzyme activity and metabolic rate, and they are required for normal skeletal and nervous system development in fetuses and infants.

TSH (thyroid-stimulating hormone) from the pituitary controls the process. It raises iodine uptake into the thyroid and stimulates hormone production and release. When iodine is scarce, TSH stays elevated and the gland can enlarge into a goiter as it tries to trap more iodine (NIH ODS). The final conversion of T4 to active T3 depends on selenium-containing enzymes; see Selenium.

How it works in the body

Iodide is absorbed quickly and almost completely in the stomach and duodenum. Iodate is reduced in the gut and absorbed as iodide. The thyroid concentrates what it needs, and more than 90% of dietary iodine leaves in the urine. A healthy adult with adequate intake holds about 15–20 mg of iodine, 70%–80% of it in the thyroid (NIH ODS).

The Wolff-Chaikoff effect

The thyroid has a built-in brake against iodine overload.

  • The acute effect. In 1948, Wolff and Chaikoff reported that giving rats large amounts of iodide caused a temporary drop in thyroid hormone synthesis lasting about 24 hours. The mechanism is not fully understood. It is thought to involve iodine-containing molecules that inhibit thyroid peroxidase, a key enzyme in hormone production (Leung & Braverman, Nat Rev Endocrinol 2014).
  • Escape. With continued exposure, normal hormone production resumes. In rats, this adaptation occurs alongside a marked fall in the sodium-iodide symporter (NIS), the transporter that carries iodide into the gland, which lowers iodine inside the thyroid (Eng et al., Endocrinology 1999).
  • Failure to escape. People who cannot adapt may develop iodine-induced hypothyroidism, which can be transient or permanent. This is the leading explanation, but the underlying mechanism remains unclear (Leung & Braverman 2014).

Most of the direct mechanism work is in rats. In humans, the evidence comes from clinical observation and small iodine-loading studies.

Measuring status

Urinary iodine is the standard measure because it directly reflects recent intake. WHO cut-offs apply to the median of a population, not to one person (NIH ODS):

Group Median urinary iodine indicating adequate intake
Children and nonpregnant adults 100–199 mcg/L as listed by the ODS; a 2026 UK review cites a WHO range of 100–299 mcg/L
Pregnant women 150–249 mcg/L
Lactating women Over 100 mcg/L
Severe deficiency (children and nonpregnant adults) Under 20 mcg/L

The limits for individuals are large. In a 15-month study of 22 healthy Swiss women, day-to-day variation was so high that about 10 spot or 24-hour urine samples were needed to estimate one woman’s status with 20% precision (König et al., J Nutr 2011). A 2026 UK review states that urinary iodine concentration is not intended for assessing individual patients (Taylor et al., Clin Endocrinol 2026).

TSH is also a poor marker, because it can stay in the normal range when intake is low (NIH ODS). Blood thyroglobulin, a protein made only by the thyroid, has been proposed as a more sensitive marker. A 2014 review found promise but inconsistent results in adults and pregnant women, with assay differences as a possible cause (Ma & Skeaff, Thyroid 2014).

Dietary reference intakes

The US Food and Nutrition Board set these values (NIH ODS):

Group RDA (mcg/day) UL (mcg/day)
Infants 0–6 months 110 (AI) Not established
Infants 7–12 months 130 (AI) Not established
Children 1–3 years 90 200
Children 4–8 years 90 300
Ages 9–13 years 120 600
Ages 14–18 years 150 900
Ages 19 years and older 150 1,100
Pregnancy 220 900 (ages 14–18); 1,100 (19 and older)
Lactation 290 900 (ages 14–18); 1,100 (19 and older)

The RDA (Recommended Dietary Allowance) covers nearly all healthy people. The AI (Adequate Intake) is used when evidence is too thin for an RDA. The UL (Tolerable Upper Intake Level) is the highest daily intake unlikely to cause harm in the general population. It does not apply to people receiving iodine as medical treatment under a physician’s care, and people with autoimmune thyroid disease or prior deficiency may react at lower intakes.

Other bodies differ:

  • WHO. 250 mcg/day during pregnancy and lactation (NIH ODS).
  • EFSA. An Adequate Intake of 150 mcg/day for adults and 200 mcg/day in pregnancy and lactation (EFSA NDA Panel, EFSA J 2014). The European upper limit is 600 mcg/day (Taylor et al. 2026).
  • American Thyroid Association. It recommends against daily supplements containing more than 500 mcg of iodine, apart from certain medical indications (Leung & Braverman 2014).

Food sources and deficiency

Iodine content in food is unusually variable because it depends on soil, animal feed and processing. Approximate values from the USDA, FDA and ODS database (NIH ODS):

Food Iodine per serving
White bread made with iodate dough conditioner, 2 slices 296 mcg
White bread made without iodate, 2 slices 1 mcg
Cod, baked, 3 ounces 146 mcg
Nori seaweed, dried flakes, 2 tablespoons (5 g) 116 mcg
Greek yogurt, plain nonfat, ¾ cup 87 mcg
Nonfat milk, 1 cup 84 mcg (samples ranged 38–160 mcg)
Iodized table salt, ¼ teaspoon 78 mcg
Egg, hard-boiled, 1 large 31 mcg
Soy beverage, 1 cup 3 mcg
Noniodized sea salt, ¼ teaspoon 0 mcg

About 20% of US bread products list iodate on their labels. Specialty salts such as sea, kosher and Himalayan salt are not usually iodized, and processed foods almost always use noniodized salt.

Deficiency

Iodine deficiency is the world’s most common cause of preventable intellectual disability. Goiter is usually its earliest clinical sign. Hypothyroidism appears when intake falls below about 10–20 mcg/day. In pregnancy, deficiency of that severity can cause major neurodevelopmental deficits, growth restriction, miscarriage and stillbirth. Chronic severe deficiency in the womb causes cretinism, which combines intellectual disability with deafness, spasticity and stunted growth (NIH ODS).

Groups at higher risk include pregnant women, vegans and others who eat little dairy, seafood or eggs, people who do not use iodized salt, and people in iodine-poor regions. Current figures from two high-income countries:

  • United States. NHANES surveys show a median urinary iodine of 164 mcg/L in people aged 6 and older (2007–2008), 119 mcg/L in women of reproductive age and 144 mcg/L in pregnant women (both 2007–2014). The last figure falls below the 150 mcg/L threshold. In pregnant women, those who ate no dairy in the previous day had a median of only 100 mcg/L, compared with 163 mcg/L in those who did (2001–2006) (NIH ODS).
  • United Kingdom. The 2019–2023 national survey reported a median of 82 mcg/L in women aged 16–49, with 30% of samples under 50 mcg/L. Only 28% of plant-based milk alternatives are iodine-fortified, and in a Nottingham vegan cohort the median was only 30–40 mcg/L (Taylor et al. 2026).

Supplement forms and label accuracy

Form Contents Status and context
Potassium iodide or sodium iodide supplement Often 150 mcg of iodine per dose in multivitamins; iodine-only products can exceed the UL Dietary supplement; potassium iodide absorption was 96.4% in a small study (NIH ODS)
Kelp or seaweed capsules 5–5,600 mcg per daily dose in one market survey Dietary supplement; highly variable (see below) (Aakre et al., Food Nutr Res 2021)
Potassium iodide tablets and solution for radiation emergencies 65 mg or 130 mg of potassium iodide per tablet; 65 mg/mL solution FDA-approved, nonprescription; thyroid blocking only (FDA)
Saturated potassium iodide solution (SSKI) 1 g of potassium iodide per mL Prescription; labeled as an expectorant at 300–600 mg three to four times daily; DailyMed lists this product as an unapproved drug (DailyMed)
Lugol’s (strong iodine) solution 5% iodine and 10% potassium iodide; about 6,250 mcg of iodine per drop Labeled for goiter and for preparing patients for thyroidectomy; DailyMed lists this product as an unapproved drug (DailyMed; Taylor et al. 2026)

Kelp and seaweed: what testing found

Seaweed content is hard to predict, and labels are unreliable:

Study What was tested Finding
Teas et al., Thyroid 2004 12 seaweed species from commercial and harvester sources 16 mcg/g in nori to over 8,165 mcg/g in processed kelp granules
Aakre et al. 2021 96 products on the Norwegian market, including 14 supplements 128–62,400 mcg per portion of whole seaweed. One portion or dose of 54 products exceeded the 600 mcg/day UL. Most labeled supplements contained less iodine than declared. Whole seaweed products averaged 1,713 mcg/g measured vs 553 mcg/g declared
US prenatal multivitamins, 2009, reported by Leung & Braverman 2014 25 brands using kelp as the iodine source Measured 33–610 mcg per daily dose vs 75–300 mcg labeled; 13 brands differed by more than 50%

The discrepancy runs both ways, so a kelp label can overstate or understate what a product delivers. Leung and Braverman concluded that vitamin preparations should use potassium iodide rather than kelp. A 2026 UK review reached the same view for pregnancy (Taylor et al. 2026).

Prenatal products also vary. Of 59 best-selling US prenatal supplements in 2016–2017, only 34 contained iodine, with a median of 150 mcg per daily serving and a range of 25–290 mcg (NIH ODS). FDA sets no ingredient standards for supplements, though iodine must be listed on the Supplement Facts label if it is present (Trumbo, Am J Clin Nutr 2016).

Approved medical use: radiation emergencies

FDA has approved potassium iodide (KI) as a thyroid-blocking agent. It floods the gland with nonradioactive iodine, so radioactive iodine is not taken up and is excreted in the urine instead. It protects only the thyroid and only against radioactive iodine. It is not a general radiation protectant and does not help against other radioactive materials (FDA).

FDA’s dosing depends on age and predicted radiation dose to the thyroid:

Group Predicted thyroid exposure KI dose
Adults over 40 Over 500 cGy 130 mg
Adults over 18 through 40 Over 10 cGy 130 mg
Pregnant or lactating women Over 5 cGy 130 mg
Adolescents 12 through 18 Over 5 cGy 65 mg (130 mg if over 150 lb or 70 kg)
Children over 3 through 12 Over 5 cGy 65 mg
Children over 1 month through 3 years Over 5 cGy 32 mg
Infants birth through 1 month Over 5 cGy 16 mg

Other FDA points:

  • Timing. KI works best within 3–4 hours of exposure, protects for about 24 hours and is dosed daily until the risk ends. It is not meant to be taken as a preventive before exposure.
  • Authority. The ThyroSafe label says to take it only when public officials direct.
  • More is not better. FDA says higher or more frequent doses add no protection and can cause severe illness or death.
  • Repeat dosing. FDA advises against it in pregnant women and neonates unless other protective measures are unavailable, because KI can suppress thyroid function in the fetus and newborn.

The evidence comes from Chernobyl rather than randomized trials. A case-control study of 276 thyroid cancer patients and 1,300 controls, all under 15 at the time of the accident, found that radiation-related thyroid cancer risk was about three times higher in iodine-deficient areas (RR 3.2; 95% CI 1.9–5.5). Potassium iodide use reduced that risk by a factor of 3 (RR 0.34; 95% CI 0.1–0.9) (Cardis et al., J Natl Cancer Inst 2005).

Poland gave at least one dose to about 10.5 million children and 7 million adults. FDA’s guidance reports mostly mild side effects: gastrointestinal upset in up to 2% of children and rash in about 1% of children and adults. Among 3,214 newborns, 12 (0.37%) had transient increases in TSH and decreases in free T4 (FDA guidance 2001).

Other medical uses

Lugol’s solution is used before thyroid surgery for Graves’ disease, with mixed evidence. A 2019 meta-analysis of 9 comparative studies with 510 patients found less thyroid blood flow and blood loss but no difference in operating time or complications (Tsai et al., Otolaryngol Head Neck Surg 2019). A later randomized trial of 56 patients found that 7 days of Lugol’s lowered free T3 and free T4 but did not reduce blood loss (median 80.5 vs 94 mL), vascularity or complications (Schiavone et al., Br J Surg 2024).

Iodine also lowers hormone release quickly in severe hyperthyroidism and thyroid storm (Leung & Braverman 2014). The effect wears off, and the strong iodine label warns that hyperthyroidism may return as severe as before or worse with continued use (DailyMed).

What the research shows

Pregnancy and child development

Severe deficiency clearly harms the developing brain. Whether supplements help when deficiency is mild is much less certain.

Study Design Result
Levie et al., J Clin Endocrinol Metab 2019 Observational; 6,180 mother-child pairs from 3 European cohorts Curved association of maternal iodine with verbal IQ, present only up to 14 weeks of gestation. Iodine-to-creatinine under 150 mcg/g was not significantly linked to lower verbal IQ (−0.6 point; 95% CI −1.3 to 0.1)
Gowachirapant et al., Lancet Diabetes Endocrinol 2017 Double-blind RCT; 832 women in India and Thailand; 200 mcg/day or placebo until delivery; 313–315 children assessed at 5–6 years No effect on verbal IQ (difference −0.7; 95% CI −2.9 to 1.5), performance IQ (−1.6; 95% CI −4.5 to 1.3) or executive function
Dineva et al., Am J Clin Nutr 2020 Systematic review of 37 publications; meta-analysis of 2 RCTs in mild-to-moderate deficiency No effect on child cognitive (−0.18; 95% CI −1.22 to 0.87), language or motor scores. Most trials showed lower maternal thyroglobulin but no change in TSH or free T4
Harding et al., Cochrane 2017 14 studies; 11 trials with over 2,700 women contributed data Too little data for meaningful conclusions. Less postpartum hyperthyroidism (RR 0.32; 95% CI 0.11–0.91) and more digestive intolerance in one small trial, both low or very low quality

Dineva and colleagues concluded that good-quality evidence is insufficient to support current supplementation recommendations in areas of mild-to-moderate deficiency. They called for trials in moderately deficient women that account for iodine intake before conception, so the question is not closed. The Cochrane authors note that their findings may not apply to severe deficiency.

Guideline bodies still recommend supplementation. The ODS summarizes the American Thyroid Association’s position as 150 mcg/day of iodine as potassium iodide for women planning pregnancy, pregnant or lactating, with similar advice from the American Academy of Pediatrics (NIH ODS). The American Thyroid Association issued new guidelines in 2026 that include iodine recommendations (Korevaar et al., Thyroid 2026). We could read only the abstract, so we cannot say whether that figure changed.

More is not safer in pregnancy. In a Spanish cross-sectional study, supplement doses of 200 mcg/day or more were linked to elevated TSH compared with doses under 100 mcg/day (NIH ODS). Neonatal hypothyroidism has been reported in infants whose mothers ate seaweed soup or drank soy milk made with kombu (Crawford et al., Med J Aust 2010; Ilmer et al., JCEM Case Rep 2026).

Cognition in children

Chronic moderate-to-severe deficiency is estimated to lower IQ by about 12–13.5 points. In a randomized trial of 184 mildly deficient New Zealand children aged 10–13 (median urinary iodine 63 mcg/L), 150 mcg/day for 28 weeks improved perceptual reasoning and overall cognitive score compared with placebo (NIH ODS). This was a single trial in a deficient group. It did not test children with adequate iodine.

Weight loss, metabolism and “thyroid support”

Thyroid hormones set metabolic rate, so iodine is often marketed for energy and fat loss. Needing iodine to make those hormones does not mean that extra iodine speeds metabolism. The trials do not support the claim:

  • Metabolic rate. In a double-blind trial of 36 healthy euthyroid adults, 4 weeks of kelp capsules raised TSH but did not change basal metabolic rate (Clark et al., Endocr Pract 2003).
  • High-dose regimens. Online “iodine loading” and “thyroid reset” protocols promote 12.5–50 mg/day. A 2026 UK review states that controlled studies show no metabolic benefit of high-dose iodine in euthyroid people and a substantial risk of hypothyroidism and autoimmune activation (Taylor et al. 2026).
  • Kelp fiber, not iodine. In a randomized trial of 50 overweight Japanese adults, iodine-reduced kelp powder (6 g/day, about 3 g of the fiber alginate, 1.03 mg of iodine) for 8 weeks lowered body fat percentage in men. There was no effect in women or in the overall per-protocol analysis, and weight change in men was not significant (P = 0.065). The authors attributed the effect to alginate, and the study was supported by a fisheries association, with one author employed by a kelp processor (Aoe et al., Mar Drugs 2021).
  • Correcting deficiency. In 163 iodine-deficient, overweight Moroccan women, 200 mcg/day for 6 months lowered TSH by 33% relative to placebo and reduced the share with high cholesterol (21.5% vs 34.8%). This shows the effect of repleting a deficiency, and the abstract reports no weight benefit (Herter-Aeberli et al., J Nutr 2015).

We found no placebo-controlled trial showing that iodine itself causes fat loss in people who are not deficient. Case reports do describe harm from kelp slimming products, including hyperthyroidism followed by overt hypothyroidism in a 45-year-old with no prior thyroid disease, and hyperthyroidism with a 6 kg weight loss in a 70-year-old woman (Di Matola et al., BMJ Case Rep 2014; Gherbon et al., Medicine 2019). In the second case, the weight loss accompanied thyroid disease.

“Thyroid support” products raise a separate issue. When researchers tested 10 such supplements, 9 contained measurable T3 (1.3–25.4 mcg per tablet) and 5 contained T4 (5.77–22.9 mcg per tablet). At the recommended dose, 5 delivered more than 10 mcg/day of T3, and some exceeded common treatment doses for hypothyroidism (Kang et al., Thyroid 2013). Any resulting effect may come from hormone rather than iodine. Prescription thyroid hormones are covered under T4 (thyroxine) and T3 (liothyronine).

Fibrocystic breast pain

Small trials suggest a possible benefit, but only at doses well above the UL. In a 111-woman placebo-controlled trial, 3,000 or 6,000 mcg/day for 5 months reduced breast pain, tenderness and nodularity, while 1,500 mcg/day did not; no major adverse events or thyroid test changes were reported. The ODS says more research is needed and that doses of this size belong under physician guidance (NIH ODS).

Side effects and risks

Excess iodine is generally well tolerated in healthy people. Risk rises with existing thyroid disease, in older adults, and in fetuses and neonates, and specific risk factors differ by outcome (Leung & Braverman 2014):

Outcome Who is at higher risk
Iodine-induced hypothyroidism Autoimmune thyroid disease; prior surgery, radioiodine or antithyroid drugs for Graves’ disease; subacute or postpartum thyroiditis; hemithyroidectomy; interferon-alpha therapy; lithium use; fetuses and neonates
Iodine-induced hyperthyroidism (Jod-Basedow phenomenon) Nodular or diffuse goiter; latent Graves’ disease; long-standing iodine deficiency

Hypothyroidism and raised TSH

Trial and cohort data show effects that can begin below the US UL:

  • Dose-finding trial. In a 4-week double-blind trial of 256 euthyroid Chinese adults, supplements of 0–2,000 mcg/day raised TSH in all iodine groups compared with placebo. Subclinical hypothyroidism (raised TSH with normal thyroid hormones) appeared in 5% at 400 mcg/day and 15%–47% at 500–2,000 mcg/day. The authors cautioned against total intake above about 800 mcg/day in China (Sang et al., Am J Clin Nutr 2012).
  • Regional cohorts. Five-year follow-up of 3,018 people in Chinese regions with different intakes found higher rates of subclinical hypothyroidism and autoimmune thyroiditis with higher iodine (Teng et al., N Engl J Med 2006):
Median urinary iodine at the regional level Overt hypothyroidism Subclinical hypothyroidism Autoimmune thyroiditis
84 mcg/L (mildly deficient) 0.2% 0.2% 0.2%
243 mcg/L (more than adequate) 0.5% 2.6% 1.0%
651 mcg/L (excessive) 0.3% 2.9% 1.3%
  • Long-term follow-up. In a 20-year prospective Chinese cohort of 1,176 people, those with iodine excess at baseline and negative thyroid antibodies had more mild subclinical hypothyroidism than those with continuous sufficiency (9.4%–12.3% vs 5.3%). The authors concluded that TSH elevation persisted even after intake fell (Gao et al., Thyroid 2026). This is observational and cannot prove cause.

Hyperthyroidism

Excess iodine can give an overactive thyroid raw material for making too much hormone, and the result may be transient or permanent (Leung & Braverman 2014). The same pattern appears when iodine reaches a previously deficient population. In Denmark, thyrotoxicosis incidence rose by about 40% in the first 3 years of iodized salt, mainly in older adults with multinodular goiter, and the rise was temporary (Taylor et al. 2026).

Kelp and seaweed foods

Double-blind trials found measurable TSH rises even at modest doses. Seaweed capsules providing 475 mcg/day for 7 weeks raised mean TSH from 1.69 to 2.19 microU/mL in 25 postmenopausal women (Teas et al., J Med Food 2007). In Australia, a kombu-containing soy milk was linked to a series of adult thyroid cases and a case of neonatal hypothyroidism (Crawford et al. 2010).

Iodism, allergy and acute poisoning

Long-term or high-dose iodide can cause iodism. Labels list burning in the mouth and throat, severe headache, metallic taste, sore teeth and gums, head-cold symptoms, swollen eyelids, excess salivation and acne-like skin lesions. Stomach upset, diarrhea and salivary gland swelling are common with drug doses (DailyMed). Hypersensitivity can cause angioedema (deep tissue swelling) and fever with joint pain.

The potassium iodide label warns against use with known iodine allergy, dermatitis herpetiformis, hypocomplementemic vasculitis, or nodular thyroid disease with heart disease (DailyMed). FDA notes that a seafood or shellfish allergy does not necessarily mean iodine allergy. Acute poisoning is rare and usually takes doses of many grams. It causes burning of the mouth, throat and stomach, fever, vomiting, weak pulse and coma (NIH ODS).

Thyroid cancer

The data are observational. Chronic deficiency may be associated with follicular thyroid cancer (NIH ODS). Some long-term studies in China and Denmark recorded more papillary thyroid cancer after iodine supplementation programs (Leung & Braverman 2014).

Pregnancy

The SSKI label warns that drug-dose potassium iodide can cause fetal harm, abnormal thyroid function and goiter, and that it passes into breast milk, where it may cause rash and thyroid suppression in the infant (DailyMed). Cases of congenital hypothyroidism have followed excess iodine tablets taken in pregnancy (Leung & Braverman 2014).

Interactions

Drug Interaction Source
Amiodarone Contains 37.3% iodine by weight, or about 75 mg per 200 mg tablet. Can cause hypothyroidism (up to 10% on the label) or hyperthyroidism (about 2%). Rates vary with background iodine: thyrotoxicosis 9.6% where intake is low, hypothyroidism 22.0% where it is normal. Contraindicated with iodine hypersensitivity Amiodarone label; Leung & Braverman 2014
Lithium Concentrates in the thyroid and inhibits hormone synthesis and release. The label warns that iodide preparations used together may produce hypothyroidism Lithium label
Antithyroid drugs (such as methimazole) High-dose iodine can add to the effect and may cause hypothyroidism NIH ODS
ACE inhibitors (such as lisinopril) Potassium iodide raises the risk of hyperkalemia (high blood potassium) NIH ODS; SSKI label
Potassium-sparing diuretics (such as spironolactone) Same concern. The SSKI label warns that hyperkalemia, arrhythmia or cardiac arrest may result NIH ODS; SSKI label

Two points add context. The hyperkalemia warning concerns the potassium in the drug, and it appears on a label where a single dose is 300–600 mg of potassium iodide. The ODS does not state a dose threshold, and we did not find reports of hyperkalemia from the microgram amounts in a multivitamin. For potassium itself, see Potassium.

Amiodarone is less a conventional interaction than a large iodine source in its own right. The 2018 European Thyroid Association guideline separates type 1 amiodarone-induced thyrotoxicosis, a form of iodine-induced hyperthyroidism, from type 2, a destructive thyroiditis (Bartalena et al., Eur Thyroid J 2018).

Effects on thyroid function tests

Iodine can change test results, which matters when reading them:

  • Small shifts within the normal range. In 32 euthyroid patients given 250–1,500 mcg/day for 14 days, the 1,500 mcg group had small falls in T4 and T3 and a small compensating rise in TSH, all within normal limits (Leung & Braverman, Curr Opin Endocrinol Diabetes Obes 2012).
  • Reversible kelp effects. In the 4-week kelp trial, TSH rose at both doses, while total T3 fell and the TSH response to stimulation increased at the high dose. Values returned to baseline within 2 weeks of stopping, except TSH in the high-dose group, which was significantly decreased (Clark et al. 2003).
  • Radioiodine uptake. The strong iodine label states that uptake measurements are “rendered useless” if iodine is given. The SSKI label states that thyroid function tests may be altered by iodide (strong iodine label; SSKI label). Patients preparing for radioactive iodine treatment usually follow a diet limited to 50 mcg/day or less for 1–2 weeks to increase uptake (NIH ODS).
  • Medical iodine loads. A CT contrast dose can contain up to 13,500 mcg of free iodine. In a small US study, urinary iodine took a median of 43 days to return to baseline (Leung & Braverman 2014).
  • Amiodarone. The label says it blocks T4-to-T3 conversion and may raise T4, lower T3 and raise reverse T3 in people with normal thyroid function (Amiodarone label).
  • Hormone in supplements. T3 or T4 in “thyroid support” products can independently shift results (Kang et al. 2013).

Because many of these sources are not obvious, Leung and Braverman advise suspecting iodine excess when thyroid dysfunction has no clear cause.

In the US, iodine is sold as a dietary supplement and added to iodized salt, while potassium iodide for radiation emergencies is an FDA-regulated drug. As of FDA’s March 2022 list, approved nonprescription products were iOSAT 65 mg and 130 mg tablets, ThyroSafe 65 mg tablets and a 65 mg/mL oral solution (FDA).

The EU permits food claims that iodine contributes to normal thyroid hormone production, energy-yielding metabolism, cognitive function, nervous system function and skin (Regulation (EU) No 432/2012). These describe what normal intake supports and are not claims of weight loss or a benefit from extra iodine.

Iodine is not named on the WADA 2026 Prohibited List.

Key takeaways

  • Essential in tiny amounts. The adult RDA is 150 mcg/day, with 220 mcg/day in pregnancy and 290 mcg/day in lactation. The UL is 1,100 mcg/day in the US and 600 mcg/day in the EU.
  • Harm in both directions. Deficiency causes goiter and developmental harm, while excess can cause hypothyroidism or hyperthyroidism, especially with existing thyroid disease.
  • Pregnancy evidence is unsettled. Severe deficiency is clearly harmful, but trials in mild deficiency have shown no child cognitive benefit. Guideline bodies still recommend supplements.
  • Kelp is unreliable. Supplements in one market survey ranged from 5 to 5,600 mcg per daily dose, and many labels were wrong. Reviews favor potassium iodide as the iodine source.
  • No weight-loss evidence. Kelp did not change basal metabolic rate in a controlled trial, and “thyroid support” products have been found to contain actual T3 and T4.
  • Potassium iodide’s emergency role is narrow. It protects only the thyroid and only against radioactive iodine, at doses set by age and exposure and directed by public officials.
  • Check medication overlap. Amiodarone is itself a large iodine source, lithium adds to hypothyroid risk, and drug-dose potassium iodide can raise potassium with ACE inhibitors and spironolactone.

Sources

  1. National Institutes of Health Office of Dietary Supplements. Iodine: fact sheet for health professionals. Updated November 5, 2024. NIH ODS.
  2. EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA). Scientific opinion on dietary reference values for iodine. EFSA J. 2014;12(5):3660. PubMed.
  3. Taylor PN, Prentice M, Bath S, et al. What endocrinologists should know about iodine: population deficiency, individual excess and misinformation in the United Kingdom. Clin Endocrinol (Oxf). 2026;105(5):523–530. PubMed.
  4. Leung AM, Braverman LE. Consequences of excess iodine. Nat Rev Endocrinol. 2014;10(3):136–142. PubMed.
  5. Leung AM, Braverman LE. Iodine-induced thyroid dysfunction. Curr Opin Endocrinol Diabetes Obes. 2012;19(5):414–419. PubMed.
  6. Eng PH, Cardona GR, Fang SL, et al. Escape from the acute Wolff-Chaikoff effect is associated with a decrease in thyroid sodium/iodide symporter messenger ribonucleic acid and protein. Endocrinology. 1999;140(8):3404–3410. PubMed.
  7. König F, Andersson M, Hotz K, et al. Ten repeat collections for urinary iodine from spot samples or 24-hour samples are needed to reliably estimate individual iodine status in women. J Nutr. 2011;141(11):2049–2054. PubMed.
  8. Ma ZF, Skeaff SA. Thyroglobulin as a biomarker of iodine deficiency: a review. Thyroid. 2014;24(8):1195–1209. PubMed.
  9. Aakre I, Solli DD, Markhus MW, et al. Commercially available kelp and seaweed products - valuable iodine source or risk of excess intake? Food Nutr Res. 2021;65. PubMed.
  10. Teas J, Pino S, Critchley A, et al. Variability of iodine content in common commercially available edible seaweeds. Thyroid. 2004;14(10):836–841. PubMed.
  11. Trumbo PR. FDA regulations regarding iodine addition to foods and labeling of foods containing added iodine. Am J Clin Nutr. 2016;104(Suppl 3):864S–867S. PubMed.
  12. Kang GY, Parks JR, Fileta B, et al. Thyroxine and triiodothyronine content in commercially available thyroid health supplements. Thyroid. 2013;23(10):1233–1237. PubMed.
  13. US Food and Drug Administration. Frequently asked questions on potassium iodide (KI). Content current as of March 24, 2022. FDA.
  14. US Food and Drug Administration. Guidance: potassium iodide as a thyroid blocking agent in radiation emergencies. December 2001. FDA.
  15. ThyroSafe (potassium iodide tablets USP, 65 mg) label. BTG International Inc. DailyMed.
  16. Potassium iodide oral solution, USP (saturated) label. Emerald Therapeutics, LLC. DailyMed.
  17. Strong iodine solution, USP (Lugol’s solution) label. Safecor Health, LLC. DailyMed.
  18. Cardis E, Kesminiene A, Ivanov V, et al. Risk of thyroid cancer after exposure to 131I in childhood. J Natl Cancer Inst. 2005;97(10):724–732. PubMed.
  19. Tsai CH, Yang PS, Lee JJ, et al. Effects of preoperative iodine administration on thyroidectomy for hyperthyroidism: a systematic review and meta-analysis. Otolaryngol Head Neck Surg. 2019;160(6):993–1002. PubMed.
  20. Schiavone D, Crimì F, Cabrelle G, et al. Role of Lugol solution before total thyroidectomy for Graves’ disease: randomized clinical trial. Br J Surg. 2024;111(8):znae196. PubMed.
  21. Levie D, Korevaar TIM, Bath SC, et al. Association of maternal iodine status with child IQ: a meta-analysis of individual participant data. J Clin Endocrinol Metab. 2019;104(12):5957–5967. PubMed.
  22. 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. Lancet Diabetes Endocrinol. 2017;5(11):853–863. PubMed.
  23. 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. Am J Clin Nutr. 2020;112(2):389–412. PubMed.
  24. Harding KB, Peña-Rosas JP, Webster AC, et al. Iodine supplementation for women during the preconception, pregnancy and postpartum period. Cochrane Database Syst Rev. 2017;3(3):CD011761. PubMed.
  25. Korevaar TIM, Leung AM, Alexander EK, et al. American Thyroid Association 2026 guidelines for thyroid disease in preconception, pregnancy, and postpartum. Thyroid. 2026;36(5):481–544. PubMed.
  26. Crawford BA, Cowell CT, Emder PJ, et al. Iodine toxicity from soy milk and seaweed ingestion is associated with serious thyroid dysfunction. Med J Aust. 2010;193(7):413–415. PubMed.
  27. Ilmer S, Lopez-Cabreza V, Salemi P. Transient neonatal hypothyroidism due to maternal seaweed soup ingestion: a report of 3 cases. JCEM Case Rep. 2026;4(10):luag263. PubMed.
  28. Clark CD, Bassett B, Burge MR. Effects of kelp supplementation on thyroid function in euthyroid subjects. Endocr Pract. 2003;9(5):363–369. PubMed.
  29. Aoe S, Yamanaka C, Ohtoshi H, et al. Effects of daily kelp (Laminaria japonica) intake on body composition, serum lipid levels, and thyroid hormone levels in healthy Japanese adults: a randomized, double-blind study. Mar Drugs. 2021;19(7):352. PubMed.
  30. Herter-Aeberli I, Cherkaoui M, El Ansari N, et al. Iodine supplementation decreases hypercholesterolemia in iodine-deficient, overweight women: a randomized controlled trial. J Nutr. 2015;145(9):2067–2075. PubMed.
  31. Di Matola T, Zeppa P, Gasperi M, et al. Thyroid dysfunction following a kelp-containing marketed diet. BMJ Case Rep. 2014;2014:bcr2014206330. PubMed.
  32. Gherbon A, Frandes M, Lungeanu D, et al. Transient hyperthyroidism following the ingestion of complementary medications containing kelp seaweed: a case-report. Medicine (Baltimore). 2019;98(37):e17058. PubMed.
  33. Sang Z, Wang PP, Yao Z, et al. Exploration of the safe upper level of iodine intake in euthyroid Chinese adults: a randomized double-blind trial. Am J Clin Nutr. 2012;95(2):367–373. PubMed.
  34. Teng W, Shan Z, Teng X, et al. Effect of iodine intake on thyroid diseases in China. N Engl J Med. 2006;354(26):2783–2793. PubMed.
  35. Gao X, Li W, Gu Q, et al. Chronic iodine excess and aging synergistically impact thyrotropin elevation: a prospective 20-year follow-up study in China. Thyroid. 2026;36(4):420–430. PubMed.
  36. Teas J, Braverman LE, Kurzer MS, et al. Seaweed and soy: companion foods in Asian cuisine and their effects on thyroid function in American women. J Med Food. 2007;10(1):90–100. PubMed.
  37. Amiodarone hydrochloride tablets USP label. RemedyRepack Inc. DailyMed.
  38. Lithium carbonate tablets label. PD-Rx Pharmaceuticals, Inc. DailyMed.
  39. Bartalena L, Bogazzi F, Chiovato L, et al. 2018 European Thyroid Association (ETA) guidelines for the management of amiodarone-associated thyroid dysfunction. Eur Thyroid J. 2018;7(2):55–66. PubMed.
  40. European Commission. Commission Regulation (EU) No 432/2012 establishing a list of permitted health claims made on foods, other than those referring to the reduction of disease risk and to children’s development and health. EUR-Lex.
  41. World Anti-Doping Agency. The 2026 Prohibited List. WADA 2026 Prohibited List.