Iodine

Iodine is an essential trace mineral whose sole known role in humans is as a structural component of thyroid hormones thyroxine (T4, tetraiodothyronine) and triiodothyronine (T3). Through these hormones, iodine influences basal metabolic rate, growth, development, thermogenesis, and fetal neurodevelopment. Historically, iodine deficiency was the leading preventable cause of intellectual disability worldwide; universal salt iodization programs have reduced this burden dramatically over the past 50 years.

Thyroid hormone synthesis

Dietary iodide is absorbed in the small intestine, reaches systemic circulation, and is concentrated by thyroid follicular cells via the sodium/iodide symporter (NIS). Inside follicles, iodide is oxidized and incorporated into tyrosine residues of thyroglobulin via thyroid peroxidase, producing mono- and diiodotyrosines that couple to form T4 (80%) and T3 (20%). These hormones are stored in colloid and released by proteolysis under TSH regulation. Peripheral T4-to-T3 conversion by selenium-dependent deiodinases further modulates hormonal activity.

Requirements

The 2001 NAM DRI set RDAs of 150 µg/day for adults, 220 µg/day during pregnancy, and 290 µg/day during lactation (reflecting breast milk iodine loss). Tolerable Upper Intake Level is 1100 µg/day, above which thyroid dysfunction (both hyper- and hypothyroidism, depending on individual thyroid status) can result. Children's RDAs increase with age from 90 µg/day for ages 1-8 to adult levels.

Deficiency spectrum

Iodine deficiency produces a graded spectrum. Mild deficiency (urinary iodine 50-100 µg/L) — diffuse thyroid enlargement (goiter), subclinical hypothyroidism, possible subtle cognitive effects. Moderate deficiency (UI 20-50 µg/L) — overt hypothyroidism, growth retardation, impaired cognition. Severe deficiency (UI <20 µg/L) — endemic cretinism, with severe intellectual disability, deaf-mutism, and motor disorders in affected children. The developmental consequences of maternal iodine deficiency during pregnancy and early infancy are permanent and motivated the global public health response.

Universal salt iodization

The 1990 UNICEF-WHO initiative for universal salt iodization (USI) established iodized salt as the dominant global strategy for iodine adequacy. Typical iodization levels of 20-40 mg iodine per kg salt, delivered through regular dietary salt consumption, provide approximately 150 µg iodine per day at typical salt intakes. As of 2026, approximately 88% of global households consume iodized salt per UNICEF data. In most iodine-sufficient countries, subclinical deficiency still affects specific populations (vegans, restrictive eaters, pregnant women, individuals using non-iodized specialty salts).

US iodine sources

Per USDA FoodData Central and published analyses: iodized salt (1/4 teaspoon) approximately 70 µg, dairy (8 oz milk) 50-100 µg (from feed supplements and teat-dip sanitizers), seaweed (1 sheet nori) 20-50 µg, seaweed (kombu, wakame) can exceed 1000 µg per serving, cod (3 oz) 100 µg, shrimp (3 oz) 35 µg, eggs (1 large) 25 µg, bread (1 slice) 40 µg (from iodate dough conditioners, variable), Greek yogurt (6 oz) 75 µg.

Current US concerns

Despite historical adequacy, several trends have produced concern about re-emerging iodine insufficiency in the US: (1) shift from iodized table salt to non-iodized specialty salts (sea salt, kosher salt, pink Himalayan salt); (2) processed food dominance, since food manufacturers generally use non-iodized salt; (3) reduced dairy consumption in plant-based diets without compensating iodine sources; (4) reduced use of iodate dough conditioners in commercial bread; (5) avoidance of iodized salt due to sodium reduction campaigns without explicit iodine substitution. NHANES data show US median urinary iodine concentrations have declined but remain within WHO-defined adequacy for the general population. Pregnant women are the subgroup of greatest concern.

Pregnancy supplementation

The American Thyroid Association recommends 150 µg supplemental iodine daily for women who are pregnant, lactating, or planning pregnancy. Most but not all US prenatal vitamins contain iodine; women should verify. Excessive iodine (>500 µg/day) during pregnancy can also cause fetal thyroid dysfunction.

Excessive iodine

Chronic high iodine intake from excessive seaweed consumption (particularly kombu) or iodine-containing medications (amiodarone) can precipitate iodine-induced hypothyroidism (Wolff-Chaikoff effect) or hyperthyroidism (Jod-Basedow phenomenon). Seaweed should be consumed moderately, particularly varieties with very high iodine content.