Medullary carcinoma arises from the parafollicular C cells of the thyroid which are neuroectodermal in origin.Hence it may be associated with other tumours of neuroectodermal origin as in MEN Ha and MEN llb.Most medullary carcinomas are located in the middle and upper thirds of the thyroid lobes which are derived from the ultimobranchial bodies having C cells.

It is the third most common of all thyroid cancers (about 5-8%) .Medullary carcinoma can be sporadic (more common) or familial.The latter may be associated with MEN type HA and JIB or without any endocrinopathy.parafollicullar cells secrete calcitonin and a carcinoembyonic antigen .Levels calcitonin have been used in the diagnosis of medullary carcinoma and in postsurgical follow up for recurrent or residual tumours.

Physiology: Calcitonin is secreted by the parafollicular c. cells of the thyroid gland .Calcitonin participates in calcium and phosphours metabolism . Most evidence indicates that calcitonin is of very little physiological importance to humans . Its primary physiological effects is to lower serum calcium levels . It does this , in part , by inhibiting bone resorption and by promoting urinary clearance of calcium3.various forms of Calcitonin (CT) may be detected in blood samples ,including a CT monomer ,an oxidized monomer , a dimmer , higher moleculer weight forms and possibly precursor of CT. The concentrations of these peptide vary with clinical status, renal function and tissular origin of CT4. Rather, calcium and phosphates homeostasis is primary under the control of parathyroid hormone (PTH ). Calcitonin reduces blood calcium levels in three ways: decreasing calcium absorption by the intestines, decreasing osteoclast activity in bones, decreasing calcium and phosphates resorption by the kidney tubules.