Iodine

Iodine is the one micronutrient where the supplement industry's more-is-better instinct is actively dangerous. The thyroid needs it to make T4 and T3 — four iodine atoms per T4 molecule — and requirements are met by roughly 150 mcg/day in adults. Above that, the relationship between intake and thyroid health becomes U-shaped rather than linear.

Deficiency remains the leading preventable cause of intellectual disability worldwide, and even mild deficiency in pregnancy is associated with lower child IQ. In iodine-sufficient countries, salt iodisation largely solved the problem, but risk has crept back among people using non-iodised sea salt or pink salt, avoiding dairy and seafood, or following vegan diets — and pregnancy raises requirements by roughly 50% precisely when the fetal brain is most sensitive.

Excess is the underappreciated half. High iodine intake triggers the Wolff-Chaikoff effect, an acute autoregulatory shutdown of thyroid hormone synthesis that most people escape within days but that can become persistent, causing hypothyroidism. In people with nodular goitre or latent Graves' disease, an iodine load can instead cause the Jod-Basedow phenomenon and precipitate hyperthyroidism. High intake also increases thyroid autoantibody titres and can accelerate Hashimoto's thyroiditis. This is why the popular high-dose protocols — 12.5 mg or 50 mg of Lugol's-style iodine, often 100 times the RDA — are genuinely risky and not supported by evidence.

Practically: 150 mcg/day covers adults, 220 mcg in pregnancy, 290 mcg while breastfeeding, and prenatal vitamins should contain potassium iodide. Kelp is unreliable, with iodine content varying by orders of magnitude between batches. Anyone with existing thyroid disease should supplement only under medical guidance.