Parathyroid Glands

• Through their secretion of parathyroid hormone (PTH):
  • The parathyroid glands are primarily responsible for:
    • Maintaining extracellular calcium concentrations
• Hyperparathyroidism:
  • Is a disease characterized by excessive secretion of parathyroid hormone:
    • An 84–amino acid polypeptide hormone
• The secretion of parathyroid hormone:
  • Is regulated directly by the plasma concentration of ionized calcium
• The main effects of parathyroid hormone (PTH):
  • Are to increase the concentration of plasma calcium:
    • By increasing the release of calcium and phosphate:
      • From bone matrix
    • Increasing calcium reabsorption:
      • By the kidney
    • Increasing renal production of:
      • 1,25-dihydroxyvitamin D-3 (calcitriol):
        • Which increases intestinal absorption of calcium
  • Thus, overproduction of parathyroid hormone results in:
    • Elevated levels of plasma calcium
• Parathyroid hormone also causes phosphaturia:
  • Thereby decreasing serum phosphate levels
• Hyperparathyroidism is usually subdivided into:
  • Primary, secondary, and tertiary hyperparathyroidism

• Usually, four parathyroid glands are situated posterior to the thyroid gland:
  • A small number of patients have 3, 5, or, occasionally, more glands:
    • The glands are identified based on their location as right or left and superior or inferior.
• The inferior glands are derived from:
  • The third pharyngeal pouch:
    • This structure is also the embryologic origin of the thymus:
      • Therefore, the inferior glands originate more cephalad than the superior glands:
        • But they migrate along with the thymus to finally become situated more inferiorly than the superior glands
      • Because of their embryologic association with the thymus:
        • The inferior glands are often found adjacent to or within the thymus
        • They are usually located near the inferior pole of the thyroid
• The superior glands are more consistent in location:
  • Usually found just superior to the intersection of the inferior thyroid artery and the recurrent laryngeal nerve
  • The superior glands are derived embryologically from the fourth pharyngeal pouch:
    • This structure also gives rise to the C cells of the thyroid gland
  • Because of their embryologic origin, the superior glands are occasionally found within the substance of the thyroid gland

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