Iodine (potassium iodide, kelp)
Summary
Iodine in supplements is mostly potassium or sodium iodide; potassium iodide was almost completely absorbed in a small study, and kelp products are also sold. Correcting mild-to-moderate deficiency improved some cognitive scores in schoolchildren, but supplementing mildly deficient pregnant women did not change child IQ at age 5-6. The adult UL is 1,100 mcg/day.
Key facts
- Potassium iodide and sodium iodide (most supplements), kelp (seaweed), iodized salt[1]
- Potassium iodide was 96.4% absorbed in a small study; seaweed iodine 61.5–101% depending on seaweed and iodine status (grade D)[1][7]
- Cognition in iodine-deficient children ↑ (B); child IQ after supplementing mildly deficient pregnant women ↔ (no-effect)[4][5][2]
- 150–200 mcg/day in pregnancy and childhood trials; one trial used a 400 mg iodized-oil dose[2][6][5]
- RDA 150 mcg; 220 mcg in pregnancy, 290 mcg while breastfeeding. UL 1,100 mcg/day[1]
- Antithyroid drugs (methimazole); potassium iodide with ACE inhibitors or potassium-sparing diuretics (high potassium)[1]
- 3 graded human outcomes on this page
- October 8, 2026
How it works
EstablishedIodine is an essential part of the thyroid hormones thyroxine (T4) and triiodothyronine (T3), which regulate protein synthesis, enzyme activity and metabolic rate and are required for skeletal and nervous-system development in fetuses and infants. When iodine is low, TSH stays high and the thyroid enlarges (goiter) as it tries to trap more iodine (established).[1]
What the human research says
Human trials only, one row per outcome. The form column shows what each trial actually tested. Grades follow our evidence-grading policy.
| Outcome | Effect | Grade | Best evidence | PMID |
|---|---|---|---|---|
| Information processing and problem solving in moderately deficient schoolchildren | Improved | Grade B | RCT · n = 310 · 2006 Iodine-deficient children aged 10-12 in rural Albania (median urinary iodine 43 microg/L; 87% with goiter) Form: oral iodized oil · Dose: 400 mg iodine as oral iodized oil · Duration: 24 weeks Iodine improved iodine and thyroid status and performance on 4 of 7 cognitive and motor tests (information processing, fine motor skills and visual problem solving) vs placebo. | 16400058 (opens PubMed)Source [5] |
| Cognitive scores in iodine-deficient children | Improved | Grade B | Systematic review · n = 166 · 2026 Children and adolescents in randomized trials of iodine vs placebo (cognition analysis: 1 study) Form: iodine supplements, oral or injected, alone or with other micronutrients · Dose: Varied across trials · Duration: Varied across trials Iodine slightly increased cognitive function vs placebo (SMD 0.19; 95% CI 0.04 to 0.34), high-certainty evidence. Weight showed little to no difference (low certainty). | 42720098 (opens PubMed)Source [4] |
| Child IQ and executive function after supplementing mildly deficient pregnant women | No effect | No effect | RCT · n = 313 · 2017 Mildly iodine-deficient pregnant women in Bangalore, India and Bangkok, Thailand; children assessed at 5-6 years Form: iodine tablets · Dose: 200 μg iodine/day · Duration: From early pregnancy (mean 10.7 weeks) until delivery No difference in verbal IQ (difference -0.7; 95% CI -2.9 to 1.5), performance IQ (-1.6) or executive function. Adverse events did not differ between groups. | 29030199 (opens PubMed)Source [2] |
Forms
| Form | Elemental % | Absorption (human data) | GI tolerance | Studied for |
|---|---|---|---|---|
| Potassium iodide (KI) | Varies[1] | 96.4% absorbed in a small study (one healthy man as the control).[7][1] | No form-specific tolerance data in our sources.[1] | Pregnancy and child trials; the form the American Thyroid Association names for prenatal use |
| Sodium iodide | Varies[1] | No head-to-head human data in our sources.[1] | No form-specific tolerance data in our sources.[1] | Common supplement form |
| Kelp and other seaweed | Varies[1] | 61.5–101% depending on the seaweed and the person's iodine status, in two groups of nine adults.[7] | No form-specific tolerance data in our sources.[1] | Bioavailability |
Studied doses
Doses studied in human trials ranged from 150 to 200 mcg/day of iodine in children and pregnant women, over about 6 to 7 months. One trial in deficient children used a 400 mg iodized-oil product. The adult UL is 1,100 mcg/day.[6][2][5][1]
Intake reference: Adult RDA: 150 mcg/day; 220 mcg in pregnancy and 290 mcg while breastfeeding. WHO recommends 250 mcg/day in pregnancy. Adult UL: 1,100 mcg/day.[1] The UL does not apply to people receiving iodine for medical treatment under a physician's care. People with autoimmune thyroid disease or prior deficiency may react to intakes considered safe for others. NIH ODS fact sheet (updated November 5, 2024).
Interactions
Published interactions only: each row cites a source. No row means none was found in our sources, not that a combination is safe.
Medicines
- Antithyroid medicinesNIH fact sheetHigh iodine doses with antithyroid drugs can have an additive effect and could cause low thyroid function.[1]
- ACE inhibitorsNIH fact sheetPotassium iodide with ACE inhibitors can raise the risk of high blood potassium.[1]
- Potassium-sparing diureticsNIH fact sheetPotassium iodide with these diuretics can raise the risk of high blood potassium.[1]
Who should be careful
- High intakes
- Too much iodine can cause some of the same problems as too little, including goiter, raised TSH and low thyroid function, and can also trigger an overactive thyroid or thyroid inflammation. The adult UL is 1,100 mcg/day; some iodine-only supplements exceed it.[1]
- Thyroid conditions
- People with autoimmune thyroid disease, or who were previously iodine deficient, may have adverse effects at intakes considered safe for the general population.[1]
- Pregnancy
- ODS reports that the American Thyroid Association and American Academy of Pediatrics advise 150 mcg/day of iodine before and during pregnancy and while breastfeeding, while noting a Spanish study linking ≥200 mcg/day supplements in pregnancy to raised TSH. Discuss amounts with a prenatal clinician.[1]
- Before you start
- Talk to your pharmacist or clinician before adding this supplement, especially if you take prescription medicines, are pregnant or breastfeeding, or have a chronic health condition.[1]
Talk to your pharmacist or clinician before starting a supplement, especially if you take prescription medicines.
Articles about Iodine (potassium iodide, kelp)
Plain-English answers to common questions, built on the evidence above.
Frequently asked questions
Which form of iodine is in most supplements?
Potassium iodide or sodium iodide; many multivitamins contain 150 mcg. Potassium iodide was almost completely absorbed (96.4%) in a small study. Kelp supplements are also sold, but seaweed iodine content and absorption vary.[1][7]
Does iodine in pregnancy raise a child's IQ?
Not in mildly deficient women so far: a large trial in India and Thailand found no difference in child IQ at age 5-6 with 200 mcg/day, and a 2020 review found too little good-quality evidence either way. Professional groups still advise prenatal iodine, as ODS reports.[2][3][8][1]
Does iodine help children's thinking?
In children who were iodine deficient, trials found modest gains on some cognitive tests, and a 2026 Cochrane review rated a slight benefit as high certainty. There is no evidence in our sources for children who already get enough iodine.[4][5][6]
Can I get too much iodine?
Yes. Above the adult UL of 1,100 mcg/day, iodine can disturb thyroid function in either direction. Some iodine-only and kelp products contain high amounts, sometimes above the UL.[1]
Sources
- [1]Iodine: Fact Sheet for Health Professionals. NIH Office of Dietary Supplements, 2024. fact-sheet
- [2]Effect of iodine supplementation in pregnant women on child neurodevelopment: a randomised, double-blind, placebo-controlled trial.. Lancet Diabetes Endocrinol, 2017. RCT
- [3]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. systematic-review
- [4]Iodine supplementation for preventing iodine deficiency disorders in children and adolescents.. Cochrane Database Syst Rev, 2026. systematic-review
- [5]Iodine supplementation improves cognition in iodine-deficient schoolchildren in Albania: a randomized, controlled, double-blind study.. Am J Clin Nutr, 2006. RCT
- [6]Iodine supplementation improves cognition in mildly iodine-deficient children.. Am J Clin Nutr, 2009. RCT
- [7]Bioavailability of seaweed iodine in human beings.. Cell Mol Biol (Noisy-le-grand), 2002. PK-study
- [8]The effect of iodine supplementation in pregnancy on early childhood neurodevelopment and clinical outcomes: results of an aborted randomised placebo-controlled trial.. Trials, 2015. RCT
What changed
- First draft. Every evidence-row PMID checked with NCBI E-utilities (esummary + efetch abstract) and re-read in an independent adversarial pass.