Parathormoni

Parathormoni, also known as parathyroid hormone (PTH), is a peptide hormone produced by the chief cells of the parathyroid glands. It regulates calcium and phosphate homeostasis, helping to maintain stable levels of ionized calcium in the blood, which are essential for neuromuscular function and bone mineralization.

PTH is synthesized as part of a larger precursor and secreted in response to low serum ionized calcium. Its release is controlled by the calcium-sensing receptor on parathyroid cells, and it has a short circulating half-life. The hormone acts on multiple organs to raise calcium levels and to balance phosphate.

In bone, PTH stimulates osteoblasts to express RANKL, which promotes osteoclast formation and bone resorption, releasing calcium and phosphate into the bloodstream. In the kidney, PTH increases calcium reabsorption in the distal tubules, decreases phosphate reabsorption in the proximal tubules (promoting phosphate excretion), and stimulates 1α-hydroxylase to form calcitriol (active vitamin D), which enhances intestinal absorption of calcium and phosphate. The net effect is an increase in serum calcium and a coordinated adjustment of phosphate metabolism.

Clinically, abnormal PTH signaling leads to disorders such as primary hyperparathyroidism (usually from a parathyroid adenoma), secondary hyperparathyroidism (often due to chronic kidney disease or vitamin D deficiency), and hypoparathyroidism (often post-surgical or autoimmune). PTH levels, together with calcium and phosphate measurements, aid diagnosis. PTH can be measured as intact hormone, with fragments more prominent in chronic kidney disease.

Management includes surgical treatment for hyperparathyroidism, vitamin D supplementation, and agents such as calcimimetics for secondary forms. PTH analogs like teriparatide are used in osteoporosis with intermittent dosing to promote bone formation.