What are the results of the Parathyroid Hormone (PTH) lab test?

From the Guidelines

Parathyroid hormone (PTH) lab testing should be used to evaluate calcium metabolism disorders and parathyroid gland function, with the most recent guidelines suggesting that therapeutic decisions should be based on trends in serum PTH levels instead of one-time values 1. The normal range for intact PTH is typically 10-65 pg/mL, though reference ranges may vary slightly between laboratories. This test should be ordered alongside serum calcium levels for proper interpretation, as PTH and calcium have an inverse relationship in normal physiology. Elevated PTH with high calcium suggests primary hyperparathyroidism, while elevated PTH with low calcium indicates secondary hyperparathyroidism (often due to vitamin D deficiency or kidney disease). Low PTH with low calcium may suggest hypoparathyroidism. For accurate results, the blood sample should be collected in the morning after fasting for 8-12 hours, and the specimen should be handled carefully as PTH is unstable at room temperature. Certain medications like lithium, thiazide diuretics, and calcium or vitamin D supplements can affect results, so these should be noted when ordering the test. PTH testing is particularly important in evaluating osteoporosis, kidney stones, chronic kidney disease, malabsorption disorders, and unexplained bone pain or fractures. Key considerations in interpreting PTH results include:

• The type of PTH assay used, as different generations of assays may recognize different fragments of PTH and have varying degrees of standardization 1
• The clinical context, including the presence of kidney disease, vitamin D deficiency, or other conditions that may affect PTH levels 1
• The need for trend monitoring, as a single PTH measurement may not accurately reflect the patient's condition 1
• The potential for interactions between PTH and other biochemical components of chronic kidney disease-mineral and bone disorder (CKD-MBD) 1. In patients with CKD, PTH levels should be interpreted in conjunction with serum phosphate and calcium levels, and treatment decisions should be based on a comprehensive assessment of these parameters 1. Overall, PTH lab testing is a valuable tool for evaluating and managing calcium metabolism disorders, but requires careful consideration of the clinical context and potential limitations of the assay.

From the FDA Drug Label

At study entry, 66% of the patients were receiving vitamin D sterols and 93% were receiving phosphate binders. Cinacalcet (or placebo) was initiated at a dose of 30 mg once daily and titrated every 3 or 4 weeks to a maximum dose of 180 mg once daily to achieve an iPTH of ≤ 250 pg/mL Plasma iPTH levels were measured using the Nichols IRMA.

The parathyroid hormone (PTH) lab used to measure iPTH levels in the study is the Nichols IRMA.

• The target iPTH level is ≤ 250 pg/mL.
• Cinacalcet is titrated to achieve this target iPTH level 2.

From the Research

Parathyroid Hormone Lab

• Parathyroid hormone (PTH) is a hypercalcemic hormone that acts on kidneys, bone, and intestine, promoting calcium release from the bone, renal calcium reabsorption, and phosphate excretion, and conversion of 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D-3 3.
• Hyperparathyroidism consists of PTH elevation, which may be adapted (secondary hyperparathyroidism) or non-adapted to calcemia levels (primary hyperparathyroidism, familial hypercalcemia/hypocalciuria, tertiary hyperparathyroidism) 3.
• Primary hyperparathyroidism (PHP) features hypercalcemia and elevated or inappropriate PTH elevation, and may be revealed by biological abnormalities such as hypercalcemia, and can be accompanied by renal complications and/or osteoporosis 3, 4.
• The diagnosis of PHP can be made through laboratory tests, including measurement of serum calcium and PTH levels, and may require additional tests such as calcium loading to diagnose normocalcemic PHP 3, 5.
• Medical management of PHP may include optimization of calcium and vitamin D intake, antiresorptive therapy for skeletal protection, and cinacalcet, a calcimimetic agent, to lower serum calcium and PTH levels 4.
• Evaluation and management of elevated PTH levels in normocalcemic patients require exclusion of causes of secondary hyperparathyroidism and consideration of medical therapy to manage symptoms and prevent complications 5, 6.