Iodine (urinary)

Iodine is a trace element the body cannot make; it must come from the diet — mainly from iodised salt, dairy, seafood, eggs and some vegetables (depending on soil iodine). Its sole essential role is as the raw material for thyroid hormones (T4 and T3), which control metabolism, growth and, crucially, brain development in the fetus and infant. Because most dietary iodine is excreted in the urine, the urinary iodine concentration is the standard way to gauge intake — though it reflects recent intake and swings day to day, so it is most reliable for assessing a population or group rather than diagnosing one person. Too little iodine forces the thyroid to work harder, enlarging into a goitre and eventually causing hypothyroidism; in pregnancy, deficiency can impair the baby's brain development and is the leading preventable cause of intellectual disability worldwide. Salt iodisation has made severe deficiency rare in many countries, but mild deficiency persists, including in some pregnant women and people avoiding dairy and iodised salt. Excess iodine (from supplements, seaweed or certain medicines and contrast dyes) can also disturb the thyroid, so both ends of the range matter.

• High iodine intake — from supplements, kelp/seaweed, iodine-rich medicines or contrast dye.
• Can disturb the thyroid — excess iodine may trigger either an under- or over-active thyroid in susceptible people.
• Recent contrast imaging — can raise iodine markedly for weeks.

• Dietary iodine deficiency — low intake of iodised salt, dairy or seafood; the leading preventable cause of goitre and hypothyroidism worldwide.
• Pregnancy and breastfeeding — higher needs make deficiency more likely and more consequential for the baby.
• Restrictive diets — avoiding dairy, iodised salt and seafood (e.g. some plant-based diets).
• Living in an iodine-poor region — low-soil-iodine areas.