see also:

• calcium
• parathyroid hormone (PTH)
• vitamin D

• Calcitonin is a 34AA polypeptide (MW 3500) synthesised and secreted by the parafollicular (or clear cells) cells of the thyroid and in small amounts elsewhere:
• “Calcitonin gene” ⇒ preprocalcitonin (thyroid) ⇒ calcitonin ⇒ katacalcin
  • ⇒ calcitonin-gene related peptide (G\CGRP) (brain);
• Plasma half life = 10 min.

• increased by:
  • proportional relationship to increased plasma [Ca] above 2.38mM;
  • beta 2 adrenergic agonists
  • dopamine
  • oestrogens
  • gastrin (thus incr. in Z-E synd.: & pernicious anaemia);
  • CCK, glucagon, secretin - none as potent as gastrin;

• inhibits osteoclasts & bone resorption;
• increases Ca renal excretion;
• THUS: ⇒ decr. [Ca] & decr. [P]

• Thyroidectomy does not significantly alter bone density or [Ca] as long as parathyroid glands are intact ⇒ exact physiologic role of calcitonin is still uncertain;
  • ? protects agains post-prandial hypercalcaemia;
  • ? protects from XS Ca loss in pregnancy and lactation;
• Medullary Ca thyroid causes very high [calcitonin] but no symptoms due to the hormone!