Iodine

Iodine is the raw material for thyroid hormone. Too little causes goitre and, in pregnancy, harms fetal brain development; too much can cause either an underactive or an overactive thyroid. People usually look it up because of thyroid concerns, pregnancy, or kelp and 'thyroid support' products.

Supplemental iodine is usually potassium iodide or comes from dried seaweed. Seaweed products are the least predictable because natural iodine content varies enormously between species and batches.

The thyroid takes up iodide and uses it to build thyroxine (T4) and triiodothyronine (T3), the hormones that set metabolic rate.

Correcting deficiency at population level prevents goitre and protects neurodevelopment, which is why salt iodisation is a public-health measure. Adequate intake in pregnancy is supported by authoritative guidance. There is no evidence that extra iodine helps people who already have enough, and case series and trials document thyroid dysfunction from excess.

Laboratory and animal studies characterise iodide transport, thyroid peroxidase activity and autoimmune responses to iodine excess.

Evidence gap: Non-human evidence, used to explain mechanism and risk rather than to support supplement benefits.

Iodine is absorbed well and cleared by the kidneys. It interacts with thyroid medicines and with tests and treatments that use radioactive iodine. Goitrogens in raw brassicas and soy matter mainly when iodine intake is low.

Excess iodine can cause both underactive and overactive thyroid, thyroiditis and, at high doses, gastrointestinal symptoms and metallic taste. Risk is higher with existing autoimmune thyroid disease or thyroid nodules. Kelp and 'thyroid support' blends are a common route to unintentionally high intake. Pregnancy needs adequate but not excessive intake, and children's requirements are much lower than adult doses. No iodine product is universally safe; appropriateness depends on baseline intake and thyroid status.

Iodised salt, prenatal multivitamins, potassium iodide tablets and kelp products are the usual forms. Prenatal products commonly include iodine in a measured amount, which is preferable to unquantified seaweed.

Adequate iodine is a nutritional adequacy issue, best met by iodised salt, dairy, eggs and seafood, with measured supplementation in pregnancy where intake is low. Iodine does not treat hypothyroidism caused by autoimmune disease, which requires thyroid hormone replacement and monitoring.

Educational example: an adult with Hashimoto's disease starts a kelp-based 'thyroid support' supplement and thyroid tests worsen at the next check. The decision factors are baseline iodine sufficiency, existing thyroid disease, and whether the product states iodine content per serving at all. Questions for a clinician or pharmacist: does my prenatal or multivitamin already contain iodine, how much is in this product, and should thyroid tests be repeated? Marketing that treats iodine as universally beneficial ignores that excess causes the very problems it claims to prevent.