Thyroid hormone control and secretion. Thyrotropin-releasing hormone
(TRH) is produced in the hypothalamus and acts on the anterior pituitary to stimulate synthesis and release of thyrotropin (thyroid stimulating hormone, TSH).
TSH stimulates synthesis and release of thyroxine (T4) and tri-iodothyronine (T3) from the thyroid gland. Over 99% of T4 and T3 in the blood are bound to thyroid-binding globulin (TBG). The remaining free thyroid hormone is responsible
for the physiologic effects of the hormone through its action at intracellular receptors, and also feeds back to reduce TRH and TSH production by the hypothalamus
and pituitary. Most of the circulating T3 (which is also an active hormone) is produced by metabolism of T4 in peripheral tissues; this process is subject
to inhibition or stimulation by a number of factors and causes circulating T3 levels to vary. Although T3 is more potent than T4 in laboratory systems,
most patients' clinical status correlates better with T4 levels than T3 levels. T3 is metabolized peripherally to inactive forms and ultimately excreted.