Thyroid function.

Normal thyroid status is dependent on the presence of many trace elements for both the synthesis and metabolism of thyroid hormones. Iodine is most important as a component of the hormones, thyroxine and 3,3',5-tri-iodothyronine (T3) and iodine deficiency may affect approximately one billion people throughout the world. Selenium is essential for normal thyroid hormone metabolism being involved with selenium-containing iodothyronine de-iodinases that control the synthesis and degradation of the biologically active thyroid hormone, T3. Additionally, selenoperoxidases and thioredoxin reductase protect the thyroid gland from peroxides produced during the synthesis of hormones. The roles of iron, zinc and copper in the thyroid are less well defined but sub- or supraoptimal dietary intakes of all these elements can adversely affect thyroid hormone metabolism.