endocrine and metabolic disorders
Overview of Parathyroid Function
Introduction
There are typically four parathyroid glands. As the name implies, they are located near the thyroid gland in the neck, although the number and particularly the location are quite variable. These pea-sized glands play a vital role in maintaining the body's calcium level. (See also Hypercalcemia and Parathyroid cells manufacture, store, and secrete a polypeptide hormone calledparathyroid hormone(PTH). PTH has several actions, but perhaps the most important is to Increase serum calcium Parathyroid cells sense decreases in serum calcium and, in response, release preformed PTH into the circulation within minutes of a fall in serum calcium concentration. PTH increases serum calcium acutely by Increasing renal and intestinal absorption of calcium Rapidly mobilizing calcium and phosphate from bone by stimulating bone resorption Renal calcium excretion generally parallels sodium excretion and is influenced by many of the same factors that govern sodium transport in the proximal tubule. However, PTH enhances distal tubular calcium reabsorption independently of sodium. PTH also Decreases renal phosphate reabsorption and thus increases renal phosphate losses By this mechanism PTH causes plasma phosphate to fall as it raises calcium thus preventing precipitation ofcalcium phosphatein body tissues both function as important regulators of bone growth and bone remodeling (see also Vitamin D Deficiency and Dependency ). PTH is rapidly cleared from the circulation by the liver, which cleaves the intact peptide into amino and carboxy terminal fragments. These fragments are then excreted by the kidneys. Radioimmunoassays for these fragments were the first tests available for diagnosing primary hyperparathyroidism and monitoring hyperparathyroidism secondary to renal disease, but because the PTH breakdown rate varies with calcium level and renal excretion can be decreased when advanced chronic kidney disease is present, second-generation assays that measure the intact PTH molecule are used. PTH increases urinary cyclicadenosinemonophosphate (cAMP). Nephrogenous cAMP excretion is measured to diagnose pseudohypoparathyroidism. 