Iodine
Iodine's only known biological role is as a component of the thyroid hormones T3 and T4. Salt iodisation was one of the great 20th-century public health interventions — eliminating endemic goitre and preventing millions of cases of iodine-deficiency brain damage. Adequacy sits between too-little (goitre, cretinism) and too-much (thyroid dysfunction) in a narrower window than most people realise.
Daily requirements
- Adults: 150 µg/day.
- Pregnancy: 220 µg/day.
- Lactation: 290 µg/day.
- Tolerable Upper Intake Level: 1,100 µg/day for adults — narrower for children.
Pregnancy — the critical window
Severe maternal iodine deficiency in early pregnancy causes irreversible cognitive impairment (endemic cretinism) in the fetus. Even mild-to-moderate deficiency correlates with lower child IQ. All prenatal vitamins should contain ~150 µg iodine; many don't — check the label. The American Thyroid Association recommends women planning pregnancy or breastfeeding supplement 150 µg/day of potassium iodide.
Goitre and hypothyroidism
Chronic iodine deficiency causes the thyroid gland to enlarge (goitre) as it works harder to trap iodide. Severe deficiency causes hypothyroidism with fatigue, cold intolerance, weight gain, and dry skin. In children, it stunts growth and impairs neurodevelopment.
Iodine excess — the Wolff-Chaikoff and Jod-Basedow effects
Acute large doses briefly shut down thyroid hormone production (Wolff-Chaikoff effect) — usually protective and self-limited. Chronic excess can either provoke autoimmune hypothyroidism (Hashimoto's) or, in people with underlying nodular thyroid disease, trigger hyperthyroidism (Jod-Basedow effect). Kelp supplements can contain 500–3,000+ µg per serving; some batches wildly exceed the label claim. Don't stack kelp with iodised salt and a multivitamin.
Are you getting enough?
Not automatically. The trend toward "sea salt," "pink Himalayan salt," and non-iodised "kosher salt" is quietly reversing decades of iodisation progress. Non-dairy milk alternatives, absent egg intake, and low seafood intake compound the problem. Australian, UK, and pregnant-women surveys have all documented rising mild iodine deficiency.
Food sources
- Seaweed, kelp (1 g dried): 232 µg (wildly variable — up to several mg)
- Cod (3 oz): 158 µg
- Iodised table salt (¼ tsp): 76 µg
- Milk (1 cup): 56 µg
- Yoghurt (1 cup): 75 µg
- Egg (1 large): 26 µg
- Shrimp (3 oz): 35 µg
- Tuna, canned (3 oz): 17 µg
- Prunes (5): 13 µg