Thyroid and nutrition
The gland in the neck that produces the hormones regulating metabolism, and whose function depends directly on two nutrients: iodine and selenium.
The thyroid produces two hormones, T4 and T3, which regulate the speed of metabolism, body temperature, heart rate and bowel transit. It is one of the few glands whose function depends directly on a dietary supply.
Iodine is part of the very structure of thyroid hormones: without it, production is impossible. Its deficiency remains the leading cause worldwide of goitre and of preventable developmental problems. Sources are few: seafood, seaweed, iodised salt, and to a lesser degree dairy products and eggs.
Selenium is the forgotten partner. This is the most useful point: the enzymes that convert T4, the storage form, into T3, the active form, are selenoproteins. A correct iodine intake is therefore not enough if selenium is lacking. Two Brazil nuts cover daily needs, and their content is such that you should not overdo them.
Two final caveats. Cruciferous vegetables (cabbage, broccoli, turnip) contain goitrogenic compounds, but their effect is negligible at usual amounts and with a correct iodine intake: there is no reason to avoid them. And too much iodine, from overconsumption of seaweed or supplements, can disturb the thyroid as much as a deficiency.