The thyroid hormones, L-3,5,3′,5′-tetraiodothyronine (L-thyroxine [T₄]) and, to a much less extent, L-3,5,3′-triiodothyronine (L-triiodothyronine [T₃]) are synthesized by the follicular epithelial cells of the thyroid gland. This synthesis requires the availability of iodine and is increased by thyroid-stimulating hormone (thyrotropin; TSH) from the anterior pituitary gland through a specific thyroidal receptor.

Small amounts of thyroid hormone are secreted continuously into the blood and are almost entirely bound (in a large circulating reservoir) to plasma proteins (Table 30-1), with a very small percentage remaining unbound or free. The speculation is that the free hormones (free T₄ and T₃), principally derived from circulating T₄, enter the cell and mediate their effects through specific nuclear receptors (see Chap. 31), which are heterogeneously distributed among tissues. The hypo-thalamic–pituitary–thyroid axis is regulated by autocrine, paracrine, hemocrine, and environmental factors to maintain steady-state hormone economy.