Parathyroid Glands

• Characteristics
  • There are four, oval-shaped endocrine glands embedded in the posterior surface of the thyroid gland
    • Two superior parathyroid glands : located near the superior pole of the thyroid gland at the junction of cricoid and thyroid cartilages.
    • Two inferior parathyroid glands: located in the area between the inferior poles of the thyroid lobes and the superior mediastinum.
• Neurovasculature
  • Arterial supply: inferior thyroid arteries
  • Venous drainage: parathyroid veins → thyroid plexus of veins
  • Innervation: thyroid branches of the cervical ganglia

Effects of the parathyroid glands are exerted through chief cells, which produce and secrete parathyroid hormone (PTH):

PTH effects:

• PTH increases serum calcium and decreases serum phosphate
  • High extracellular calcium → activation of calcium-sensitive receptors → ↓ PTH excretion
  • Low extracellular calcium → inhibition of calcium-sensitive receptors → ↑ PTH excretion
• Increases renal activation of vitamin D:
  • Calcitriol (1,25-dihydroxyvitamin D) is the active form.
  • Increases intestinal reabsorption of calcium
• Stimulation of calcium and phosphate release from the bones through osteoclast activation

• Hyperparathyroidism : demonstrated by pathologically elevated PTH levels. Depending on the pathogenesis, the presentation may be primary, secondary, or tertiary hyperparathyroidism. The most frequent cause is adenomas of the parathyroid gland,. Hyperfunctioning adenomas are typically removed with surgery.
• Hypoparathyroidism : Parathyroid hormone deficiency results in low serum calcium levels and elevated phosphate levels. The cause of hypoparathyroidism is most commonly iatrogenic following neck surgery (e.g., thyroidectomy). Treatment is calcium replacement therapy with frequent lab monitoring to ensure appropriate physiological levels.
• Parathyroid carcinoma: accounts for approximately 1% of all cases of hyperparathyroidism (hypersecretion of PTH accounts for approximately 90% of cancers).