The Mechanism of Thyroid Hormone Secretion

SUMMARY

Release of thyroid hormones from peptide linkage in follicular colloid to the circulation is normally carefully modulated by pituitary thyrotrophin. Secretion of these hormones is now thought to occur by a sequence of intricate, coordinated changes within the stimulated cell. This sequence begins with the formation of colloid droplets derived from portions of the follicular colloid brought into the cell by pinocytosis. Lysosomes that contain hydrolytic enzymes move from the base of the cell toward the apex to fuse with these droplets. Within these new organelles, phagolysosomes, lysis of thyroglobulin occurs to release the hormonal amino acids thyroxine and triiodothyronine. Evidence for this sequence coming from both morphological and biochemical experiments has been reviewed. The possibility that derangement of this sequence plays a role in human thyroid disease has been suggested.