Calcitonin & Analogues

Calcitonin is a 32-amino acid linear polypeptide hormone that is produced in humans primarily by the parafollicular cells (also known as C-cells) of the thyroid, and in many other animals in the ultimobranchial body. It acts to reduce blood calcium (Ca2+), opposing the effects of parathyroid hormone (PTH). It has been found in fish, reptiles, birds, and mammals. Its importance in humans has not been as well established as its importance in other animals, as its function is usually not significant in the regulation of normal calcium homeostasis.

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 hormone participates in calcium (Ca2+) and phosphorus metabolism. In many ways, calcitonin counteracts parathyroid hormone (PTH). More specifically, calcitonin lowers blood Ca2+ levels in two ways:

1. Inhibits Ca2+ absorption by the intestines
2. Inhibits osteoclast activity in bones

While calcitonin is the antipode of parathyroid hormone (PTH) in those actions, it is not dissimilar from PTH in its effect on electrolyte reabsorption in the kidneys. Namely, calcitonin:

1. Inhibits phosphate reabsorption by the kidney tubules]
2. Calcitonin increases tubular reabsorption of Ca2+, leading to decreased rates of its loss in urine.

Secretion of calcitonin is stimulated by:

1. an increase in serum [Ca2+] 
2. gastrin and pentagastrin.