Parathyroid Glands and Hormone

Anatomy

  • Classically, 4 parathyroid glands lie behind the lobes of the thyroid.  They are small, bead like structures (25-40mg).  Two glands (superior and inferior) sit behind each thyroid lobe
  • Blood supply via the inferior thyroid artery (or if this is not present, by a branch of the superior thyroid artery)
  • Composed of Chief cells and oxyphils (the former produces PTH, the function of the latter is unknown)

Physiology

  • The main role of the parathyroid gland and PTH is to maintain serum calcium homeostasis.  This is a complex process involving calcium, phosphate, vitamin D and PTH.
    • More that 99% of the total body calcium is stored within bone as phosphate/hydroxide salts.  Only a small proportion can be exchanged into the plasma.
  • PTH is produced in response to low circulating calcium levels
    • With prolonged exposure, PTH stimulates osteoclast activity and proliferation- causes increased bone resorption and the release of calcium and phosphate into the serum
      • NB with pulsatile/minimal exposure, PTH actually causes new bone formation.  PTH receptors are only on osteoblasts- over activation causes secondary osteoclast activation.
    • increases calcium reabsorption at the kidneys.  It also acts at the kidneys to increase active vit D production.  This increases the amount of calcium and phosphate absorbed from the gut
      • Note that PTH actually acts to increase excretion of phosphate by the kidneys.  This actually causes a net reduction in serum phosphate levels.

Calcium_regulation

 

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