Parathyroid Hormone (PTH)

PTH is secreted in response to low calcium levels as a 118 amino acid polypeptide that undergoes two successive cleavages to yield an 84 amino acid biologically active hormone. It has a very short half-life of fewer than five minutes and breaks down into various fragments of which the biological significance remains to be defined.

Producing too much parathyroid hormone causes raised calcium levels in the blood (hypercalcemia), and this is referred to as primary hyperparathyroidism. It is most often caused by a benign tumor in one or more of the parathyroid glands, and patients with this condition have high PTH and calcium levels. Secondary hyperparathyroidism occurs in response to low blood calcium levels and is caused by other mechanisms, for example, kidney disease and vitamin D deficiency. It is often seen in patients with chronic renal failure (CRF). The kidneys fail to excrete sufficient phosphate, and the parathyroid gland secretes PTH in an effort to lower calcium levels to balance the calcium-phosphate ratio. Tertiary hyperparathyroidism is a relatively rare condition that causes hypercalcemia due to excess parathyroid hormone production on the backdrop of the glands being overactive.