Calcitonin is a 32 amino acid polypeptide hormone that is produced in humans primarily by the parafollicular (also known as C) cells of the thyroid, and in many other animals in the ultimobranchial body. Calcitonin is formed by the proteolytic cleavage of a larger prepropeptide which is the product of the CALC1 gene (CALCA). The CALC1 gene belongs to a superfamily of related protein hormone precursors including islet amyloid precursor protein, calcitonin gene-related peptide, and the precursor of adrenomedullin.

The calcitonin receptor is a G protein-coupled receptor which is coupled by G_s to adenylyl cyclase and thereby to the generation of cAMP in target cells.

The hormone participates in calcium (Ca²⁺) and phosphorus metabolism. In many ways, calcitonin has the counter effects of parathyroid hormone (PTH).  Specifically, calcitonin reduces blood Ca²⁺ levels in three ways:
[1]. Decreasing Ca²⁺ absorption by the intestines

[2]. Decreasing osteoclast activity in bones

[3]. Decreasing Ca²⁺ and phosphate reabsorption by the kidney tubules

Calcitonin also has 3 broad actions:

Bone mineral metabolism:  event postprandial hypercalcemia resulting from absorption of Ca²⁺ from foods during a meal, Promote mineralization of skeletal bone, Protect against Ca²⁺ loss from skeleton during periods of Ca²⁺ stress such as pregnancy and lactation.

Vitamin D regulation

A satiety hormone: Inhibit food intake in rats and monkeys and May have CNS action involving the regulation of feeding and appetite.