Parathyroid Hormone (PTH) is a single-chain polypeptide containing 84 amino acids. PTH raises serum ionised calcium levels through direct action on bone and the kidneys. It increases the rate of calcium ion flow from bone to the extra cellular fluid and increases both the renal tubular re-absorption of ionised calcium and the renal excretion of phosphate. Long term regulation of total body calcium by PTH occurs through it’s stimulation of Vitamin D metabolism, which results in enhanced absorption of ionised calcium.In healthy individuals PTH is secreted in response to circulating calcium ion levels. If an individual calcium level drops below normal there is a drastic increase in he PTH secretion. Calcium levels returning to normal exert a negative feedback effect, thus inhibiting PTH secretion by the parathyroid glands. PTH undergoes proteolysis to a lesser extent in the parathyroid glands but mostly peripherally to yield N terminal fragments and longer lived C terminal fragments. N terminal confers bioactivity. The intact hormone has an in vivo half life of 2 – 5 minutes.
In hypercalcaemia due to primary hyperparathyroidism or to ectopic PTH production, the majority of patients have high PTH levels. In patients with hypercalcaemia due to malignancy or other causes the PTH is typically low or within normal reference range. PTH levels are also high in secondary hyperparathyroidism usually associated with renal failure as a result of constant stimulation of the parathyroid gland by low calcium levels.
Hypocalcaemia along with a low PTH level is expected in hypoparathyroidism either post surgery or idiopathic.
1.3 – 7.3 pmol/L
A separate EDTA sample is required if a FBC is also requested
Biochemistry Lab, James Cook University Hospital, Marton Road, TS4 3BW