Parathyroid hormone (PTH) is produced and secreted by the parathyroid gland, and is regulated by Ca++and calcitriol. PTH has a very short half-life (3-5 min) in serum, and therefore a steady rate of secretion is necessary to maintain serum PTH concentrations. Factors other than Ca++, such as high serum magnesium and phosphorus concentrations, may impact PTH secretion. Markedly increased serum phosphorus concentrations (as seen in renal failure) can lower serum Ca++ concentration resulting in an increase in PTH secretion.

The rate of PTH secretion is inversely proportional to the concentration of extracellular calcium, but only over a narrow range corresponding to a total serum Ca of 7.5 to 11.0 mg/dL. As little as a 10% decrease in serum Ca++ concentration elicits a nearly maximal PTH secretory response, whereas a rapid 2-3% decrease in Ca++ concentration can produce a 400% increase in PTH secretion.

The most important biological effects of PTH on calcium are to: (1) increase blood calcium concentration, (2) increase tubular reabsorption of calcium, resulting in diminished calcium loss into the urine, (3) increase bone resorption and the numbers of osteoclasts on bone surfaces, and (4) accelerate the formation of the principal active vitamin D metabolite (calcitriol) by the kidney.

To prevent degradation of intact PTH, serum samples should be stored and shipped frozen to the laboratory. After 4 days at 4°C, only 65% of the hormone remains, and after 7 days at 24°C, only 19% remains. When frozen, however, PTH is stable for up to 2 months at -20°C. A two-site immunoradiometric (IRMA) assay for intact human PTH has been validated in the dog and cat. Serum PTH concentrations should be evaluated relative to simultaneous measurement of serum Ca++ concentration.

In primary hyperparathyroidism, both Ca++ and PTH are elevated. If parathyroid glands are normal, hypercalcemia (parathyroid-independent) should be associated with low PTH concentration, and hypocalcemia should be associated with elevated concentrations of PTH.

With renal failure and secondary hyperparathyroidism, animals will have increased serum PTH with normal or decreased Ca++ concentration.