Calcitonin is the hypocalcaemic hormone discovered by Copp in 1962. It is a 32 amino acid single polypeptide produced by parafollicular ‘C’ cells of thyroid. Parathyroids, thymus and cells of medullary carcinoma of thyroid also contain calcitonin.
Synthesis and secretion of calcitonin is regulated by calcium concentration itself. Rise in plasma calcium increases while fall in plasma calcium decreases calcitonin release.
The actions of calcitonin are generally opposite to that of parathyroid hormone. It inhibits bone resorption by direct action on osteoclasts-decreasing their ruffled surface which forms contact with the resorptive pit. It is doubtful whether it also promotes calcium deposition by osteoblasts. The hypocalcemic action of calcitonin lasts ~8 hours.Calcitonin inhibits proximal tubular calcium and phosphate reabsorption by direct action on kidney. However, hypocalcaemia which occurs overrides the direct action by decreasing the total calcium filtered filtered at the glomerulus urinary calcium is actually reduced.
The actions of calcitonin are probably mediated through increase in cAMP formation, but its target cells are different from that of PTH.
Synthetic salmon calcitoin is used clinically, because it is more potent due to slower metabolism. Human calcitonin has also been produced.
Adverse effects experienced are nausea, flushing, tingling of fingers, bad taste and allergic reaction. By lowering plasma calcium calcitonin may interfere with action of digoxin.
Hyperparathyroidism, hyper vitaminosis D, osteolytic bony metastasis. It acts rapidly, but refractoriness develops over time and other measures to reduce plasma calcium are more convenient.
100 IU s.c. or i.m. daily along with calcium and vitamin D supplements.
A nasal spray formulation delivering 200 IU per actuation has become available. One spray in one nostril daily has been shown to increase bone mineral density in menopausal women effect becoming perceptible after 6 months therapy; and maintained thereafter. It is indicated when estrogens cannot be given and the women is menopausal for at least 5 years with definite evidence of osteoporosis. Rhinitis, epistaxis, nasal ulceration and headache are side effects.
100 U daily or on alternate days produces improvement for view months. Later, resistance usually develops due to production of antibodies. Human calcitonin may prove better in this regard.