Peptide hormone secreted by parathyroid gland chief cells that controls ionized calcium levels in blood and body fluids by increasing 1,25-dihydroxyvitamin D3 (by the kidney), mobilizing calcium from bone (due to increased osteoclast activity), increasing renal tubular resorption of calcium, and reducing renal clearance of calcium, increasing intestinal calcium absorption. The half-life of PTH is <5 minutes. Ionized calcium in blood inhibits PTH secretion. Biologic activity resides in the first 34 terminal amino acids. The intact hormone has 84 amino acids but can be quickly cleaved by proteolysis into smaller less-active fragments. Assay for the intact PTH has largely superseded tests for various PTH fragments. It is important that the PTH assay not cross-react with PTH (7-84) lacking the 6-N-terminal, which has been shown to be a weak antagonist to PTH activity and may lower serum and plasma calcium levels.
Normal range: 12 " “65 pg/mL.
Use
Differential diagnosis of hyperparathyroidism and hypoparathyroidism.
Very sensitive in detecting PTH suppression by 1,25-dihydroxyvitamin D; therefore, used for monitoring that treatment of chronic renal failure.
Intraoperative PTH assay to determine removal of abnormally secreting tissue; may replace routine frozen section; can replace traditional four-gland explorations and distinguishes single from multiglandular disease.
Preoperative and 10 " “20 minutes postresection assay; this causes 50 " “75% reduction, indicating successful resection of parathyroid adenoma.
Interpretation
Increased In
Primary and secondary hyperparathyroidism
Pseudohypoparathyroidism
Hereditary vitamin D dependency types 1 and 2, vitamin D deficiency
Z-E syndrome
Familial medullary thyroid carcinoma
MEN types I, IIa, and IIb
Decreased In
Autoimmune hypoparathyroidism
Sarcoidosis
Nonparathyroid hypercalcemia in the absence of renal failure
Hyperthyroidism
Hypomagnesemia
Transient neonatal hypocalcemia
DiGeorge syndrome
Limitations
At this time, there are significant intermethod variations in PTH results for different manufacturer assays. This is mainly due to cross-reactivity with various PTH fragments of the assay.
The finding of a persistently high-normal calcium accompanied by a high-normal PTH (alternatively, a low-normal calcium accompanied by a low-normal PTH) warrants further investigation; for the PTH, although itself within normal limits, may still be inappropriately high (or inappropriately low) relative to the circulating calcium level.
Because of a pronounced nocturnal rise in intact PTH levels observed in a small experimental male population, sampling after 10 am for optimum discrimination between normals and those with mild primary hyperparathyroidism has been suggested.
Sedative " “hypnotic drug propofol (Diprivan) may give falsely low PTH values.
High concentrations of hemolysis, lipemia, and bilirubin should be avoided.
Rapid intraoperative PTH that declines ≥50% from the highest baseline in 10 minutes after resection indicates successful total excision.