1. Thyroid secretes triiodothyronine or T3 , tetraiodothyronine or thyroxine or T4 and thyr ocalcitonin.
2. The thyroid gland synthesize, store and discharge these hormones.
3. T3 and T4 are iodinated derivatives of amino acid tyrosine which are secreted by thyroid follicular cells and stored in follicles. They have similar function. The secretion of T3 and T4 are regulated by Thyroid stimulating hormone (TSH) or thyrotropin of pituitary gland in negative feedback manner. T3 is more active and T4 is more potent hormone.
4. Thyrocalcitonin is secreted by the parafollicular cells.
5. Thyrocalcitonin regulates blood calcium level. It stimulates bones to take up Ca++ from the blood and deposit it in the form of calcium phosphates in the bones, thereby decreasing blood Ca++ level. Increased calcium level of blood stimulates ‘C’ cells to secrete thyrocalcitonin and vice versa.