Regulation of thyroid hormone secretion by the thyroid begins at the level of the hypothalamus. Neuroendocrine cells in the hypothalamus release thyrotropin-releasing hormone (TRH) into the portal circulation of the pituitary. TRH stimulates the thyrotropic cells to secrete TSH (thyroid stimulating hormone). Subsequently, TSH stimulates the thyroid gland to produce thyroid hormones: 15% T3 (tri-iodothyroxine) and 85% T4 (thyroxine).
T4 is converted into T3, mostly in the liver. Thus, the liver is the main producer of T3 for the body. T4 controls the feedback loop by inhibiting the release of TRH and TSH. Thyroxine has mainly calorigenic effects (increasing metabolic rate) in peripheral tissues.
I. TSH is a peptide hormone that binds to a G-protein coupled receptor.
II. Thyroid hormones are predominantly bound to thyroxine-binding globulin in the blood.
Extra info: Like other pituitary hormones (LH and FSH), TSH is a glycoprotein: a protein with attached sugar groups. These kind of proteins bind to a G-protein coupled receptor. Thyroid hormone does not travel freely through the circulation, because it is hydrophobic. More than 99% is bound to a binding protein.