parathyroideahormon

Parathyroideahormon, commonly known as parathyroid hormone (PTH), is a peptide hormone produced by the chief cells of the parathyroid glands. It is a central regulator of calcium and phosphate homeostasis in mammals, acting on bone, kidney, and indirectly on the intestine to maintain serum calcium within a narrow range.

In bone, PTH stimulates osteoblasts to increase expression of RANKL, which promotes the formation and activity of osteoclasts. This leads to bone resorption and the release of calcium and phosphate into the bloodstream. In the kidney, PTH enhances calcium reabsorption in the distal tubules and reduces phosphate reabsorption in the proximal tubules, increasing blood calcium and promoting phosphate excretion. PTH also stimulates renal 1α-hydroxylase, increasing production of 1,25-dihydroxyvitamin D (calcitriol), which raises intestinal absorption of calcium and phosphate.

Regulation of PTH secretion is tightly linked to serum calcium levels via the calcium-sensing receptor (CaSR) on parathyroid cells. Low calcium stimulates PTH release, while high calcium suppresses it. Vitamin D status, phosphate levels, magnesium, and hormonal feedback from calcitriol and other factors modulate PTH secretion and action. FGF23 also influences phosphate and vitamin D metabolism, with indirect effects on PTH under certain conditions.

Clinically, abnormal PTH levels are central to disorders of calcium metabolism. Primary hyperparathyroidism (often from a parathyroid adenoma) causes elevated PTH and hypercalcemia. Secondary hyperparathyroidism arises from chronic hypocalcemia (such as in kidney disease). Hypoparathyroidism results in low PTH, leading to hypocalcemia and hyperphosphatemia. PTH measurement, together with calcium and vitamin D status, aids in diagnosis and management.