The thyroid gland is made up of thousands of follicles, which are small spheres lined with follicular cells. Follicular cells convert thyroglobulin, a protein found in follicles, into two iodine-containing hormones, tri-iodo-thyronine or T3, and thyroxine or T4. Once released from the thyroid gland, these hormones enter the blood and bind to circulating plasma proteins. Inside the cell T­4 is mostly converted into T3, at which point it can exert its effect. The thyroid is also made up of parafollicular or C cells, which are near the follicles. These cells produce calcitonin, a hormone that lowers blood calcium levels.