Iodine

What it does

Iodine has one overriding job: it is the raw material for thyroid hormones. The thyroid gland uses iodine to make thyroxine (T4) and triiodothyronine (T3), the hormones that set the body's metabolic rate — the speed at which cells use energy. Through these hormones, iodine influences almost every organ, affecting body temperature, heart rate, growth and how efficiently food is turned into energy.

Because thyroid hormones control growth and brain development, iodine is critically important during pregnancy and early life. A shortage during these periods can impair a child's brain development, and historically iodine deficiency was a leading preventable cause of intellectual disability worldwide. In adults, a lack of iodine can cause the thyroid to enlarge as it tries to trap more of the mineral, producing a swelling in the neck called a goitre.

Iodine is found in sea fish, shellfish, dairy products and eggs, and in many countries table salt is fortified with iodine to ensure a reliable supply. Intake can be low in people who avoid dairy and fish, including some following plant-based diets, since most plant foods contain little unless grown in iodine-rich soil. Both too little and too much iodine can upset thyroid function, so balance matters.