parathyreoideahormon

Parathyreoideahormon, also called parathyroid hormone (PTH), is a peptide hormone produced by the chief cells of the parathyroid glands. It is a key regulator of calcium and phosphate metabolism in humans, acting to raise low serum calcium and to modulate phosphate levels. PTH exerts its effects on bone, kidney, and, indirectly, the intestine via active vitamin D (calcitriol).

PTH secretion is controlled primarily by extracellular calcium through the calcium-sensing receptor (CaSR) on parathyroid cells. When calcium levels fall, PTH secretion increases; when calcium rises, PTH release is suppressed. Magnesium status and vitamin D can influence PTH levels as well.

In bone, PTH stimulates osteoblasts to express RANKL, which activates osteoclast-mediated bone resorption and releases calcium and phosphate into blood. In the kidney, PTH increases calcium reabsorption in the distal tubules and decreases phosphate reabsorption in the proximal tubules. It also upregulates 1α-hydroxylase, enhancing production of 1,25-dihydroxyvitamin D (calcitriol), which increases intestinal absorption of calcium and phosphate. The overall effect is a rise in serum calcium and a decrease in serum phosphate, with a net effect depending on hormonal levels and organ responses. PTH has a short half-life, on the order of minutes.

Clinically, abnormal PTH levels reflect parathyroid disorders. Primary hyperparathyroidism, usually due to a parathyroid adenoma, causes hypercalcemia and may present with bone loss. Secondary hyperparathyroidism arises from chronic hypocalcemia (e.g., CKD or vitamin D deficiency). Hypoparathyroidism leads to hypocalcemia and requires calcium and vitamin D supplementation. PTH assays assist in diagnosis. Treatments include surgery to remove adenomas, calcimimetics such as cinacalcet to suppress PTH release, and, in osteoporosis, PTH analogs like teriparatide to stimulate bone formation.