= Thyroid hormone
{title2=$T_3,\ T_4$}
<Thyroid hormones> are iodine-containing signaling molecules produced by the <thyroid> <gland>, principally thyroxine, T4, and triiodothyronine, T3. Their production involves iodinated residues within thyroglobulin; many target <tissues> convert T4 to the more active T3. <Thyroid hormones> help regulate metabolic activity and development.<Anatomy and Physiology 2e by J. Gordon Betts and others>[Section 17.4, Synthesis and Release of Thyroid Hormones]{ref}
Much circulating <hormone> is protein-bound. A total measured <concentration> and the free <hormone> <concentration> therefore describe different pools. Feedback through the <hypothalamus> and pituitary connects <thyroid> output to further stimulation of the <gland>. <Hormone> production, transport binding, conversion and <receptor> response are separate mechanisms; a change in one does not specify the others.
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