Calcitonin

Secretion

Secreted by parafollicular cells in the thyroid gland
* also called clear cells or C cells

Action

Lowers [Ca2+]

• Not secreted when calcium level is below ~9.5mg/dL
• Above that level, secretion is proportional to calcium level
• Inhibits bone resorption
• Increase urinary Ca2+ excretion

Lowers [PO4]

Metabolism

Halflife <10min

Secretion is increased by

• beta-adrenergic agonist
• Dopmaine
• Oestrogen
• GIT-related hormone
  * Gastrin (most potent)
  * CCK
  * Glucagon
  * Secretin

NB:

• Even though of all the GIT-related hormones, gastrin is the most potent. However, Gastrin level produced by food is insufficient to stimulate an increase in calcitonin.

Significance

[WG21:p398] The exact physiologic role of calcitonin is UNCERTAIN

• Little long-run effect on [Ca2+]
  * Patients with medullary carcinoma has very high circulating levels of calcitonin, but no symptoms and no bone changes.
  * NO syndromes relating to calcitonin has been described
• May pay a role in bone development or protect against postprandial hypercalcemia
• Can be used for Paget's disease or severe hypercalcemia