In human beings, calcitonin is produced from the parafollicular cells (C cells) of the thyroid gland. Calcitonin is made up of 32 amino acids with a molecular weight of 3500. It is a peptide hormone secreted by the parafollicular cells of thyroid gland.
Actions:
It is a hypocalcemic agent and hence decreases plasma calcium level. When thyroid gland is perfused with blood containing high calcium, secretion of calcitonin is increased. This lowers plasma calcium and phosphate levels.
Calcium lowering effect of calcitonin is brought about by its action on the bones. It prevents calcium resorption from bone. It inhibits the activity of osteoclasts and increases the excretion of calcium in urine.
Regulation of Secretion:
It is directly dependent on plasma calcium level.
Factors which influence calcitonin secretion are given in the following box:
Hypo- or hypersecretion of calcitonin is not associated with any clinical abnormality. Medullary carcinoma of thyroid gland is associated with increased production of calcitonin, but no signs and symptoms are seen in this hyperstate of secretion of calcitonin. It has been shown that comparatively more hormone is secreted in young individuals.
It may be help for deposition of calcium on bone and facilitates bone growth. Diet rich in calcium when given, the gastrin which is released in gastric phase of gastric juice secretion may stimulate the calcitonin release as well.
The increased release of calcitonin diverts calcium to bone tissue thus preventing excessive rise in plasma calcium level. It may also protect the mother from excessive drainage of calcium during pregnancy and lactation. The excessive drain of calcium during pregnancy may be due to increased level of 1, 25- DHCC.