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).
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.