Iodine

Needed for one thing only, and that thing matters: the thyroid hormones that set your metabolic rate. Requirement rises in pregnancy, when a shortage affects the developing brain irreversibly.

Its only job is thyroid hormone — and thyroid hormone sets the metabolic rate of every cell, and governs brain development before birth.

• Iodised salt
• Seaweed — extremely variable, kelp can be excessive
• Cod and haddock
• Dairy
• Eggs

150 µg · 220 µg in pregnancy for most adults. The upper limit, past which harm becomes likely, is 1,100 µg.

Reference intakes are population figures. Pregnancy, illness, age and medication all move an individual’s needs, which is one of several reasons this page is information rather than advice.

A sustained shortage has a name here: Goitre · Congenital iodine deficiency. That is worth pausing on — nutrients whose deficiency earned a name are the ones where the consequences were severe enough, historically, to be described as a disease.

Most at risk: People using only non-iodised sea or rock salt, vegans, and pregnancy.

Living where soil iodine is low without iodised salt; avoiding both dairy and seafood; and moving to non-iodised speciality salts — sea salt and pink salt contain very little. Pregnancy and breastfeeding raise requirements substantially.

Thyroid enlargement, fatigue, weight gain and cold intolerance. In pregnancy, shortage causes irreversible intellectual disability in the child — historically the world’s leading preventable cause.

Every sign here has other, often likelier, explanations. A list of symptoms is a reason to look into something, never a diagnosis.

There is no good blood test for iodine status; urinary iodine is used to assess populations rather than individuals. Thyroid function tests measure the consequence rather than the intake.

Iodised salt, dairy, eggs, and white fish. Seaweed is extremely rich and extremely variable — kelp in particular can deliver many times the upper limit in a single serving.

Prenatal supplements usually contain it, and should. Too much iodine causes thyroid problems just as too little does, so kelp tablets and high-dose iodine are not a casual purchase.

Too much also causes thyroid dysfunction — both over- and underactivity. Kelp supplements can contain wildly variable and sometimes enormous amounts. This is a nutrient where more is emphatically not better.

Interactions: Interacts with thyroid medication and lithium.