A butterfly-shaped gland in the neck that makes thyroid hormones, which set the pace of metabolism in nearly every cell.
Iodine
Thyroid hormones are built from iodine. Each T4 molecule holds four iodine atoms, T3 holds three.
Selenium
Selenium-containing enzymes convert T4 into the more active T3.
Evidence & sources
The thyroid needs iodine to make thyroid hormones. Evidence: established. Known: Basic physiology. Unknown: Both too little and too much iodine can disturb thyroid function. Source: NIH Office of Dietary Supplements, Iodine Fact Sheet for Health Professionals (Current revision).