Calcitonin
[Ref: AV6:chp10; WG21:chp21]
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