Calcitonin is produced by -

Ans. is 'a' i.e., Thyroid Calcitonin o Calcitonin is a polypeptide hormone is produced by parafollicular 'C' cell of thyroid (major source). o Parathyroids, thymus and cells of medullary carcinoma of thyroid also contain calcitonin. Synthesis and secretion of calcitonin is regulated by plasma Ca++ concentration itself - rise in plasma Ca++ increases, while fall in plasma Ca++ decreases calcitonin release. Actions: Calcitonin decreases calcium levels by : - It inhibits bone resorption by direct action on osteoclasts (Osteoclasts have receptors for calcitonin). It is the major action. Calcitonin inhibits proximal tubular calcium and phosphate reabsorption by direct action on the kidney.