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Iodine — The Complete Supplement Guide

The thyroid mineral with a narrow safe window: why it matters most in pregnancy, the goitre link, and why kelp supplements can give you far too much.

Reviewed by Bodil Isaksson, Licensed Biomedical Scientist (Sweden)Last reviewed July 2026Supplement guide

SupplementsMineralsIodine

A rare case where both too little and too much upset the same organ — the thyroid — which makes getting the amount right unusually important. For the short dietary version, see the Minerals page.

Read first. Iodine has a narrow safe range. High-dose kelp/seaweed supplements can deliver many times the safe amount and trigger thyroid dysfunction — especially risky in existing thyroid disease. Pregnancy needs are higher; discuss with a professional. Information, not a prescription.

Key takeaways

  • Both too little and too much harm the thyroid — aim for the right amount.
  • Adults need 150 µg/day (220–250 in pregnancy).
  • Upper limit 600 µg (EFSA) / 1,100 µg (US).

Function

Iodine has essentially one job — but a vital one:

  • Thyroid hormones: the essential building block of T3 and T4, the hormones that set metabolic rate in every cell.
  • Metabolism & energy: via thyroid hormones, it governs how fast the body burns energy, body temperature and heart rate.
  • Brain development: critically important in pregnancy and infancy — adequate iodine is essential for normal fetal brain growth.
  • Growth: supports normal growth and development in childhood.

Different forms — and which to choose

Potassium iodide / iodateThe standard supplement and salt-fortification forms; predictable dose.
Kelp / seaweedNatural but highly variable and often excessive iodine — easy to overshoot the safe limit.
Iodised saltThe main public-health source in many countries.
Lugol's / high-doseMedical use only (e.g. thyroid emergencies) — not for general supplementation.

Absorption in the body

How muchReadily and almost completely absorbed; the thyroid actively concentrates it, and the kidneys excrete the excess.
NoteNo large store beyond the thyroid, so steady intake matters — particularly in pregnancy.

Deficiency symptoms

SymptomsGoitre (enlarged thyroid) and hypothyroidism — fatigue, weight gain, cold intolerance, dry skin.
PregnancyDeficiency in pregnancy impairs fetal brain development and is the world's leading preventable cause of intellectual disability.
Who's at riskPeople not using iodised salt, vegans (few plant sources), and pregnant/breastfeeding women.

Recommended intake

Adults 150 µg · Pregnancy 220–250 µg

Reference intakes:

  • Adults: 150 µg/day
  • Pregnancy: ~220–250 µg/day · Breastfeeding: ~250–290 µg/day

Best sources: iodised salt, dairy, eggs, white fish and shellfish; seaweed is very rich but unpredictable.12

Therapeutic use

DeficiencyCorrecting low intake (a ~150 µg supplement, e.g. in pregnancy multivitamins where salt isn't iodised).
MedicalPotassium iodide to protect the thyroid in radiation emergencies, and before some thyroid surgery — specialist use.

Therapeutic doses

Supplement: ~150 µg/day
EverydayA 150 µg supplement covers a shortfall; pregnancy formulas typically include ~150 µg.
Avoid megadosesHigh-dose kelp or "iodine drops" are unnecessary and risky for the thyroid.

Toxicity & upper limit

UL: 600 µg (EFSA) · 1,100 µg (US)
Upper limit600 µg/day (EFSA) or 1,100 µg/day (US) for adults.1
ExcessParadoxically causes thyroid dysfunction — both hyper- and hypothyroidism — and can trigger problems in people with existing thyroid disease.
Kelp warningA single serving of some seaweed/kelp supplements can exceed the upper limit many times over (see also the Algae & sea vegetables iodine note).

Bottom line: use iodised salt and eat dairy, eggs and fish. A modest ~150 µg supplement is sensible if you avoid these or are pregnant — but skip high-dose kelp, and be cautious if you have thyroid disease. More is not better with iodine.

← All supplementsShort dietary version

References

Reference and upper-limit values are general adult figures from official bodies; individual needs vary. Therapeutic doses describe clinical/research use, not prescriptions.

  1. US NIH Office of Dietary Supplements. Iodine — Health Professional Fact Sheet. ods.od.nih.gov
  2. EFSA Panel (NDA). Dietary Reference Values for iodine. EFSA Journal 2014;12(5):3660. efsa.onlinelibrary.wiley.com

This article summarizes nutrition science from official health authorities and peer-reviewed sources for education. It is not a substitute for individual medical advice, and the doses given are not prescriptions — always consult a professional before therapeutic supplement use, in pregnancy, in kidney disease, or alongside medication.

✓
Bodil Isaksson, Licensed Biomedical Scientist (Sweden)

At Food Health Lab we explain what the science says in plain language, and we cite our sources so you can check for yourself.

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