Other Nutrients

Iodine

By All The Nutrients Editorial Team Published: 7 min read

Iodine is a trace mineral whose only confirmed biological role is as a structural component of the thyroid hormones thyroxine (T4) and triiodothyronine (T3) — yet that single function makes it indispensable for life. Thyroid hormones regulate basal metabolic rate, protein synthesis, and the development and maturation of the central nervous system. The RDA is 150 µg/day for adults, rising to 220 µg/day during pregnancy and 290 µg/day while breastfeeding. Iodine deficiency remains the leading preventable cause of intellectual disability worldwide, affecting an estimated 1.88 billion people, predominantly in low-soil-iodine regions far from the sea. See the top food sources of iodine ranked by µg per 100 g from USDA data.

Top Source

Salt, table, iodized

5080 ug

What Does Iodine Do?

  • Thyroid hormone synthesis — iodine is incorporated into thyroglobulin in the thyroid gland to produce T4 (tetra-iodothyronine) and T3 (tri-iodothyronine). These hormones bind nuclear receptors in virtually every cell and regulate metabolic rate, thermogenesis, heart rate, cholesterol clearance, and the transcription of hundreds of genes involved in growth and development.
  • Fetal and infant brain development — thyroid hormones are critical for neuronal migration, myelination, and synaptic formation during fetal development and the first two years of postnatal life. Iodine deficiency during this window causes cretinism (severe neurological impairment) and, at subclinical levels, measurably lower IQ scores — an effect studied in populations across 38 countries. Adequate maternal iodine is therefore one of the highest-priority nutritional interventions in pregnancy.
  • Metabolic rate regulation — T3 and T4 increase the expression of Na⁺/K⁺-ATPase (the sodium-potassium pump) in cell membranes, which drives 20–40% of basal metabolic rate in adults. Hypothyroidism from iodine deficiency therefore directly causes metabolic slowing, weight gain, cold intolerance, and fatigue — all reversible with adequate iodine intake.
  • Antioxidant function at high concentrations — iodide at high concentrations (as found in thyroid tissue and breast tissue) acts as a direct antioxidant, scavenging reactive oxygen species. This may help explain associations between adequate iodine status and lower risk of fibrocystic breast disease, though clinical evidence remains preliminary.

Signs of Iodine Deficiency

Iodine deficiency disorders (IDD) form a spectrum from subclinical cognitive impairment through goitre (thyroid gland enlargement) to overt hypothyroidism and cretinism. The severity depends on timing (fetal exposure is most damaging) and degree of deficiency. Deficiency is most common in landlocked regions, high-altitude areas, and wherever dietary diversity is low and iodised salt is not widely used. Subclinical deficiency is underdiagnosed because thyroid blood tests may remain within "normal" range despite suboptimal iodine status.

  • Goitre — visible or palpable thyroid gland enlargement as the gland hypertrophies to capture more circulating iodide; the most visible sign of population-level iodine deficiency
  • Hypothyroidism — fatigue, unexplained weight gain, cold intolerance, constipation, dry skin, hair thinning, slow heart rate, and low mood from insufficient T3/T4 production
  • Cognitive impairment and reduced IQ — most damaging when deficiency occurs during pregnancy or early childhood; subclinical iodine deficiency reduces IQ by an average of 12–13 points in affected populations (Bleichrodt & Born, 1994 meta-analysis)
  • Cretinism — severe, irreversible intellectual disability, short stature, deaf-mutism, and motor impairment from severe fetal iodine deficiency; now preventable through universal salt iodisation
  • Pregnancy complications — increased risk of miscarriage, preterm birth, and low birth weight in iodine-deficient mothers
  • Subclinical hypothyroidism in women — particularly common in regions without iodised salt; often misdiagnosed or undiagnosed despite causing fatigue, depression, and fertility issues

Consider supplementing

Iodine (kelp) supplement

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How to Get the Most from Iodine

Dietary iodine is rapidly and nearly completely absorbed (>90%) from the small intestine as iodide or iodate. The thyroid gland concentrates iodide to levels 20–50× those in blood plasma via the sodium-iodide symporter (NIS). Certain compounds can interfere with thyroid iodine uptake.

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Use iodised salt as your baseline source

Iodised table salt — the most effective and widely available iodine source — typically contains 45–76 µg iodine per gram of salt. One teaspoon (5 g) of iodised salt provides approximately 225–380 µg of iodine, covering the adult RDA. Sea salt and gourmet salts are not reliably iodised unless labelled. Note that reducing sodium intake by switching to non-iodised salt without other dietary iodine sources can lead to inadequacy.

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Goitrogens — food compounds that block thyroid iodine uptake

Cruciferous vegetables (cabbage, kale, broccoli, Brussels sprouts) and soy contain glucosinolates and isoflavones that inhibit the sodium-iodide symporter or interfere with thyroid peroxidase — the enzyme that organifies iodine. At typical dietary quantities alongside adequate iodine intake, these effects are not clinically significant. In people already iodine-deficient or with thyroid disease, very large habitual quantities of raw cruciferous vegetables may worsen thyroid function; cooking reduces goitrogenic activity substantially.

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Dairy and seafood provide highly bioavailable iodine

Cow's milk, yogurt, and cheese are among the most concentrated dietary iodine sources in countries like the US and UK — largely due to iodine-containing sanitisers used in dairy equipment and iodine in cattle feed. Saltwater fish (cod, tuna, shrimp) and seaweed are naturally iodine-rich. Seaweed iodine content is highly variable — kombu can contain 1,500–8,000 µg per gram, far exceeding the Tolerable Upper Intake Level (UL) of 1,100 µg/day for adults if consumed regularly.

Common Iodine Questions

What does iodine do for the body?

Iodine's primary role is as a building block of thyroid hormones T3 and T4, which regulate metabolism, body temperature, heart rate, and the development of the brain and nervous system. Without adequate iodine, the thyroid cannot produce enough hormone, leading to goitre, hypothyroidism, and — during pregnancy and early childhood — irreversible cognitive impairment.

What are the signs of iodine deficiency?

The most visible sign is goitre (thyroid enlargement). Symptoms of hypothyroidism from deficiency include fatigue, weight gain, cold intolerance, constipation, dry skin, thinning hair, slow heartbeat, and depression. Subclinical deficiency during pregnancy is particularly serious because it impairs fetal brain development without obvious maternal symptoms. A urinary iodine concentration (UIC) test is the standard way to assess population iodine status.

Which foods are highest in iodine?

Seaweed (especially kelp/kombu — variable but extremely high), cod and other white fish, dairy products (milk, yogurt, cheese), shrimp, eggs, and iodised salt are the richest sources. Iodised salt (≈76 µg/g) is the most reliable everyday source in countries that use it. Freshwater fish and most plant foods contain minimal iodine unless grown in iodine-rich soil.

How much iodine do I need per day?

The RDA is 150 µg/day for adults, 220 µg/day during pregnancy, and 290 µg/day while breastfeeding. The Tolerable Upper Intake Level (UL) for adults is 1,100 µg/day — easy to exceed with regular seaweed consumption. Most people in countries with iodised salt programmes meet the RDA through diet alone; vegans and people avoiding dairy and seafood are at higher risk of inadequacy.

Is iodine safe to take as a supplement?

Iodine supplements are safe within the RDA and moderate ranges, but excess iodine (above 1,100 µg/day chronically) can paradoxically inhibit thyroid hormone synthesis (Wolff-Chaikoff effect) and trigger or worsen autoimmune thyroid disease (Hashimoto's or Graves'). Most multivitamins contain 150 µg — appropriate for deficiency prevention. Megadose iodine supplements (often sold for "detox" or thyroid support) carry real risk and are not recommended without medical supervision.

Do vegans get enough iodine?

Vegans are at significant risk of iodine insufficiency because the richest sources (dairy, seafood, eggs) are excluded and iodine content of plant foods varies widely with soil. Studies consistently find lower urinary iodine in vegans compared to omnivores. Vegans should use iodised salt, consume seaweed occasionally (avoiding daily kelp due to excessive content), or take a supplement containing 150 µg/day of iodine.