Iodine
Potassium iodide / kelp · Foundation
Essential for thyroid hormone, and one of the few nutrients where too much causes the same disease as too little.
What it is
The mineral your thyroid is built around, and a rare case where both too little and too much cause the same problem. Iodized salt largely solved deficiency in iodine-sufficient settings, which is precisely why casual supplementation is now more likely to cause trouble than fix it.
How it works
Thyroid hormones are literally iodine atoms bolted onto tyrosine. T4 carries four, T3 carries three. Too little and the thyroid cannot build hormone (goitre, hypothyroidism). Too much can paradoxically shut hormone production down, or trigger overactivity in a susceptible thyroid. The U-shaped risk curve is the whole story.
Which form to buy
The public-health intervention that made deficiency rare. ⚠️ Note most sea salt, kosher salt and the salt in processed food is NOT iodized.
The default, and the reason most people are covered.
US guidelines recommend 150 mcg/day supplemental iodine in pregnancy, and deficiency genuinely harms fetal brain development, though trials in already-replete women have not shown better child outcomes. Not all prenatals contain it, and amounts vary widely.
The guideline-recommended supplemental case.
Iodine content is wildly variable and can be enormous, kelp products are a documented cause of thyroid dysfunction.
Unpredictable dosing of a nutrient with a narrow window.
Internet protocols use amounts hundreds of times the requirement, well past the tolerable limit.
A genuinely risky trend.
What it actually does, graded
One of the great public-health successes, iodization prevents intellectual disability at population scale.
Needs rise in pregnancy and deficiency causes irreversible harm, the reason prenatals contain it.
In iodine-sufficient settings most hypothyroidism is autoimmune (Hashimoto's), not deficiency, and extra iodine can make autoimmune thyroid disease WORSE.
A common marketing angle with no support in replete people.
Some early research in fibrocystic breast changes; not established.
Pros and cons
- Essential, with a genuinely heroic public-health record
- The pregnancy case is clear and important
- Iodized salt makes it nearly free
- Excess causes the same thyroid problems as deficiency
- Kelp supplements deliver unpredictable and sometimes enormous amounts
- Particularly hazardous in autoimmune thyroid disease, which is common
- The "everyone is deficient" internet narrative is mostly false in iodized-salt countries
What to expect
- Correcting genuine deficiency: thyroid function normalizes over weeks to months.
- Excess: paradoxically, the same symptoms deficiency causes, which is why self-dosing is a poor idea.
Interactions
Is it for you?
- Pregnancy or planning it, check your prenatal contains it
- A genuinely iodine-poor diet: no iodized salt, no dairy, no seafood, no eggs
- You regularly use iodized salt or eat dairy and seafood in decent amounts, you are likely replete
- You have Hashimoto's, extra iodine can make it worse
Who should avoid it
- Anyone with autoimmune thyroid disease, without endocrinologist direction
- People on thyroid medication or amiodarone, without prescriber input
- Anyone considering high-dose internet iodine protocols
Research on this supplement
Papers about this supplement, not proof of the claims above. A title says what was studied, which is sometimes a negative result or a different question entirely.
- Chlorhexidine or Povidone-Iodine Solution Irrigation Versus Saline Irrigation for the Prevention of Postoperative Infections in Primary Total Joint Arthroplasty: A Systematic Review and Meta-Analysis
- The Comparative Efficacy of Chlorhexidine Gluconate and Povidone-iodine Antiseptics for the Prevention of Infection in Clean Surgery: A Systematic Review and Network Meta-analysis
What the evidence says
Deficiency is the leading preventable cause of intellectual disability worldwide, and iodised salt is one of public health's great successes. In iodine-replete countries frank deficiency is uncommon, though pregnancy raises requirements. Supplementing beyond need does not improve thyroid function.
What studies used
Requirements sit near 150 mcg/day, higher in pregnancy; the tolerable upper limit is 1,100 mcg. Published studies had not established a preferred timing.
Reported from published studies and product labelling — a record of what was used in research, not a recommendation. What is right for you is a conversation with a clinician.
Watch out for
- Excess iodine causes both hyperthyroidism and hypothyroidism, and can trigger thyroiditis in susceptible people, the U-shaped curve is real
- Kelp products are the usual route to accidental megadoses; their content varies enormously and is often unlabelled
Evidence base
established literature
Large human evidence base: several phase 3 or 4 trial reports, plus multiple meta-analyses or systematic reviews.
This letter measures how much human research exists, not whether it works for you — the tier above (Strong) is the read on quality.
Research (12)
- Thyroid-Gut-Axis: How Does the Microbiota Influence Thyroid Function?Nutrients · 2020
- Effect of selenium on thyroid autoimmunity and regulatory T cells in patients with Hashimoto's thyroiditis: A prospective randomized-controlled trialClin Transl Sci · 2021
- Iodine excessBest Pract Res Clin Endocrinol Metab · 2010
- Smoking and thyroidClin Endocrinol (Oxf) · 2013
- Selenium and thyroidBest Pract Res Clin Endocrinol Metab · 2009
- Pregnancy and iodineThyroid · 2001
- [Iodine and thyroid autoimmunity]Z Arztl Fortbild Qualitatssich · 2004
- GoiterCurr Probl Surg · 1969
- Iodine and thyroid autoimmune disease in animal modelsThyroid · 2001
- Over-the-counter-drug-induced thyroid disordersEndocr Pract · 2013
- Iodine excess and thyroid autoimmunityJ Endocrinol Invest · 2003
- Correlation between iodine intake and thyroid disorders: a cross-sectional study from the South of ChinaBiol Trace Elem Res · 2014
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