The thyroid gland comprises 2 connected lobes on either side of the neck. Almost all vertebrates have thyroid glands.
The thyroid modulates how quickly the body uses energy and produces proteins. The thyroid also moderates sensitivity to certain hormones. Thus, the thyroid is instrumental in controlling metabolism and growth.
Triiodothyronine and thyroxine are the principle thyroid hormones, synthesized by addition of iodine to the amino acid tyrosine. These hormones act on nearly every cell in the body: instrumental in metabolism, body temperature, heart rate, growth, and development.
Iodine is found in the ocean. It is a relatively rare element, and the heaviest essential element with biological function. Iodine deficiency leads to cognitive malfunction.
Tyrosine is an essential amino acid that cells use to synthesize proteins. Tyrosine is prominent in proteins that participate in extracellular signaling.