Calcitonin
Also known as: thyrocalcitonin, CT
A 32-amino-acid peptide from thyroid C cells that lowers plasma calcium by inhibiting osteoclasts; important in children and as a tumour marker, but of minor physiological importance in adults.
Overview
Calcitonin is released by parafollicular cells when plasma Ca²⁺ rises (sensed by the calcium-sensing receptor) and in response to gastrin. It binds the calcitonin receptor, a Gs-coupled GPCR (also Gq) on osteoclasts: within minutes the osteoclasts retract their ruffled borders and stop resorbing bone, so less Ca²⁺ and phosphate enter the blood. In the kidney it modestly increases Ca²⁺ and phosphate excretion.
Its importance in human adults is small: neither total thyroidectomy (no calcitonin) nor medullary thyroid carcinoma (very high calcitonin) disturbs calcium balance noticeably. It may matter more during rapid skeletal growth, pregnancy and lactation, protecting the maternal skeleton.
- Location
Source: parafollicular (C) cells of the thyroid. Targets: osteoclasts, kidney.
Function
- Inhibits osteoclastic bone resorption.
- Increases renal calcium excretion.
- Lowers plasma calcium (mildly and transiently).
- Physiology
Class: peptide (32 aa). Receptor: calcitonin receptor — Gs/cAMP. Half-life ≈10 min. Escape (receptor down-regulation) occurs with continued exposure.
- Regulation
Stimulated by hypercalcaemia (CaSR) and gastrin/pentagastrin; inhibited by low calcium.
Clinical significance
Tumour marker
Serum calcitonin is used to diagnose and monitor medullary thyroid carcinoma and to screen RET-mutation carriers.
Therapeutic use
Salmon calcitonin (more potent than human) is occasionally used for rapid short-term lowering of severe hypercalcaemia; its effect wanes within 48 h (tachyphylaxis).
Test yourself
Calcitonin is secreted by which cells, and what is its main clinical use?
- Parathyroid chief cells; marker of hyperparathyroidism
- Thyroid follicular cells; marker of papillary carcinoma
- Parafollicular (C) cells; tumour marker for medullary thyroid carcinoma
- Renal tubular cells; marker of vitamin D status
Answer: Parafollicular (C) cells; tumour marker for medullary thyroid carcinoma
Neural-crest-derived C cells secrete calcitonin, which inhibits osteoclasts. Its physiological role in adults is minor, but serum calcitonin is a sensitive marker of medullary thyroid carcinoma (sporadic or MEN2).
Compiled from standard anatomy and physiology references — see the Endocrine System sources. For education only; not medical advice.