In humans, the four parathyroid glands, imbedded in the thyroid gland, secrete the parathyroid hormone (PTH). So-called C cells dispersed throughout the thyroid gland, the parathyroids and thymus secrete another hormone, calcitonin (CT). Both hormones play a significant role in the maintenance of calcium homeostasis.

A. Major actions of parathyroid hormone (PTH) and calcitonin

PTH, a polypeptide secreted in response to hypocalcemia raises the concentration of plasma calcium by :

1. increasing renal calcium reabsorption;
2. mobilizing calcium from bones by stimulating osteoclastic activity (i.e. destruction of bone cells),
3. in the presence of adequate amounts of vitamin D, stimulating the absorption of calcium from the small intestine; and
4. lowering levels of plasma inorganic phosphate by inhibiting renal reabsorption of phosphate.

Calcitonin, also a polypeptide, is secreted not only in response to calcium levels but also to gastrointestinal hormones such a glucagon. CT receptors are found in bones and the kidneys and the major actions of this hormone involve regulation of plasma calcium levels. They include:
1. lowering of plasma calcium and phosphate levels by inhibiting bone resorption;
2. increasing calcium excretion in urine
CT has also some minor action on water and electrolytes and decreases gastric acid secretion.