Parathyroid hormone, calcitonin and vitamin D are the main agents that control the homeostatic regulation of extracellular calcium, inorganic phosphate and bone growth (mineral metabolism).
In the human adult, about 98% of the calcium and 85% of phosphorus are present in the bones. So the bones are the principal reservoir of these minerals.
There occur cellular dysfunctions due to abnormalities in bone mineral homeostasis. Parathyroid hormone and vitamin D are the principal or primary regulators of calcium and phosphorus homeostasis.
Besides these minerals, metabolism is also influenced by thyroid hormones (such as calcitonin), growth hormone, androgens, oestrogens, and Glucocorticoids, which are called as secondary regulators.
Parathyroid hormone is a polypeptide. It consists of 84 amino acids. It is secreted from the parathyroid gland. Its secretion is controlled by the plasma concentration of calcium.
When the plasma levels of calcium are decreased (hypocalcaemia) parathormone increases plasma calcium by:
• Transfer of calcium from bone to blood
• Absorption of calcium from the intestines
• Reabsorption of calcium from the kidney
Further, Parathyroid hormone lowers plasma phosphate levels. The release of calcitonin is inhibited during hypocalcaemia. Vitamin D helps in the absorption of calcium.
The opposite effects are seen when serum calcium level is high (hypercalcaemia). Since hypoparathyroidism is best treated by vitamin D and calcium supplements, the clinical use of parathormone is very limited.
The secretion of Parathyroid hormone is increased during hypocalcaemia and decreased at hypercalcaemia. Its clinical use is very limited because hypoparathyroidism is best treated by vitamin-D and calcium supplements.
The secretion of calcitonin from parafollicular C cells of the thyroid gland is increased during hypercalcaemia and decreased during hypocalcaemia.
Therapeutic benefit of Parathyroid hormone is transient due to development of antibodies and down regulation of specific receptors.
Vitamin-D is used: (a) to prevent and cure rickets and osteomalacia; (b) in hypoparathyroidism; (c) in osteoporosis; (d) in Fanconi’s syndrome; and (e) in vitamin D deficiency secondary to malabsorption, liver disease, and chronic renal diseases.
Glucocorticoids are used for a short period for the treatment of hypercalcaemia and to determine the cause of hypercalcaemia.
Oestrogens are used for the prevention and/or treatment of postmenopausal osteoporosis.
Biphosphonates, pilcamycin and thiazides are non-hormonal agents which affect bone mineral homeostasis. Biphosphonates and pilcamycin are used in Paget’s disease and to treat hypercalcaemia. Thiazides are used in reducing hypercalciuria and incidence of stone formation in Parathyroid hormone.