Introduction The parathyroid glands secrete parathyroid hormone (PTH) PTH helps to maintain homeostasis of serum calcium and serum phosphate secreted by the chief cells of the parathyroid glands Anatomy the parathyroid glands are 4 small glands, < 1 cm in diameter, located immediately posterior to the thyroid gland usually located left superior, left inferior, right superior, right inferior Regulation primarily regulated by serum calcium in a negative feedback loop calcium is sensed by the parathyroid glands' extremely sensitive calcium-sensing receptor (CaSR) ↑ serum calcium results in ↓ PTH secretion ↓ serum calcium results in ↑ PTH secretion severe hyperphosphatemia will also result in ↑ PTH secretion Parathyroid Hormone Target organs PTH ultimately has effects on three organs bone kidney gut Effects net result is to ↑ serum calcium and ↓ serum phosphate mnemonic - Phosphate-Thrashing Hormone Physiologic Effects of Parathyroid Hormone By Target Organ Target Organ Effect Bone ↑ Bone resorption of calcium and phosphate Kidney ↑ Conversion of calcidiol (25-OH vitamin D) to calcitriol (1,25-(OH)2 vitamin D) ↓ Calcium excretion ↑ Phosphate excretion Gut ↑ Calcium and phosphate absorption in the small intestines due to the ↑ calcitriol Related Clinical Conditions Hypoparathyroidism Hyperparathyroidism