Illustration of the capillary blood supply of a follicle in the human thyroid gland. The thyroid endocrine gland consists of many closed spheroidal follicles, of which one is cut through and dis- played here (blue). Cuboidal epithelial cells, which secrete thyroid hormones, surround a central hormone storage chamber. Each follicle is richly supplied with blood, and a plexus (network) of capillaries (red, blood cells seen at cut ends) transports secreted thyroid hormone to the rest of the body. Lymph vessels are yellow. Thyroid hormones control metabolism, influence growth and maturation, and regulate blood calcium levels.