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.[ref]
Anatomy and Physiology 2e by J. Gordon Betts and others. Section 17.4, Synthesis and Release of Thyroid Hormones
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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