Lab Test: Parathyroid Hormone, PTH (Blood) Level
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- Measurement of parathyroid hormone (PTH) in serum or plasma for the evaluation and management of thyroid and kidney disorders. It is routinely monitored in patients with chronic renal failure (CRF).
- Adults (serum):
- C-terminal and midmolecule: 50-330 pg/mL (50-330 ng/L)
- N-terminal: 8-24 pg/mL (8-24 ng/L)
- Intact molecule: 10-65 pg/mL (10-65 ng/L)
- Adults (plasma):
- C-terminal and midmolecule: < 50 microLEq/mL (< 50 mLEq/L)
- N-terminal: < 6.1 pmol/L
- Intact molecule: 1-5 pmol/L
- Pediatrics:
- C-terminal and midmolecule (ages 1-16): 51-217 pg/mL (51-217 ng/L)
- N-terminal (ages 2-13): 14-21 pg/mL (14-21 ng/L)
- Intact molecule (ages 2-20): 9-52 pg/mL (9-52 ng/L)
- Parathyroidectomy in patients with primary hyperparathyroidism
- An intraoperative drop in PTH of >50% immediately following resection has been described as curative. Surgical cure has also been defined as a serum calcium level of <10.2 mg/dL within 6 months following parathryoidectomy.
- Hypocalcemia secondary to thyroidectomy
- A single intraoperative or postoperative intact PTH measurement may predict the development of symptomatic postoperative hypocalcemia in thyroidectomy patients, and may predict the need for vitamin D supplementation.
- Suspected hypercalcemia due to hyperparathyroidism
- In the presence of normal renal function, persistently mild hypercalcemia and elevated intact PTH levels at the upper end of the normal range are findings consistent with a diagnosis of primary hyperparathyroidism. Normal changes in PTH secretion occur over a very narrow range of serum calcium concentrations. Maximal suppression of PTH occurs at a serum calcium level of 11 to 12 mg/dL or greater; maximal secretion of PTH is stimulated with a serum calcium level of 7 to 8 mg/dL or lower. A decreased PTH level is common in most nonparathyroid hypercalcemic disorders without renal failure, including hypercalcemia relat4ed to malignancy.
- Chronic kidney disease
- an elevated PTH level in conjunction with secondary hyperparathyroidism is common in patients with chronic kidney disease, particularly in stage 3 patients with glomerular filtration rates less than 60 mL/min/1.73 m2. Treatment with small doses of vitamin D sterols improves bone mineral density without negatively impacting kidney function.
- Target ranges for patients with CKD are different than those with normal kidney function since some assays may incorrectly detect inactive fragments of PTH in CKD patients. The target ranges of plasma intact PTH by stage of CKD are as follows:
- Stage 3: 35-70 pg/mL (3.85-7.7 pmol/L)
- Stage 4: 70-110 pg/mL (7.7-12.1 pmol/L)
- Dietary phosphorus should be limited to between 800 and 1,000 mg/day when iPTH levels are above the target range for the patient's CKD stage.
- An iPTH of <400 pg/mL in the presence of low to normal serum calcium levels is consistent with mild hyperparathyroidism. iPTH >500 to 600 pg/mL suggests moderate or severe hyperparathyroidism. Parathryoidectomy may be appropriate in hypercalcemic and/or hyperphosphatemic patients with repeated iPTH levels >800 pg/mL who do not respond to standard medical interventions.
- Serum phosphorous, calcium and iPTH should be monitored in all chronic kidney disease patients with a glomerular filtration rate below 60 mL/min/1.73 m2. Patients with CKD stage 3 with a GFR of 30-59 mL/min/1.73 m2 should have their PTH levels measured every 12 months. Patients with CKD stage 4 with a GFR of 15-29 mL/min/1.73 m2 should have their PTH levels measured every 3 months. CD stage 5 patients undergoing dialysis or with a GFR <15 mL/min/1.73 m2 should have their PTH levels tested every 3 months.
- Hypocalcemia
- Elevated PTH may be found in hypocalcemia due to intestinal malabsorption, vitamin D deficiency, renal failure, and hereditary vitamin D resistance. Additionally, if PTH levels are not elevated in untreated patients with these disorders, then primary parathyroid failure should be considered as an additional diagnosis.
- Suspected or known bone disease
- Elevated PTH levels are associated with hyperparathyroid bone disease. PTH levels 2 to 3 times normal are considered necessary in order to sustain normal bone formation and to prevent adynamic bone disease in chronic kidney disease patients.
- PTH is the only hormone secreted by the parathyroid gland in response to hypocalcemia. When calcium serum levels return to normal, PTH levels diminish. PTH, therefore, is one of the major factors affecting calcium metabolism. This test is useful in establishing a diagnosis of hyperparathyroidism and distinguishing nonparathyroid from parathyroid causes of hypercalcemia.
- It is important to measure serum calcium simultaneously with PTH. Most laboratories have a PTH/calcium nomogram already made up indicating what PTH level is considered normal for each calcium level.
- Whole (intact) PTH is metabolized to several different fragments, including an animo or N-terminal, a midregion or midmolecule, and a carboxyl or C-terminal. The intact PTH and the N-terminal are metabolically active. These can all be measured by immunoassay. The intact PTH and all fragments generally provide accurate information concerning the level of PTH in the blood. The intact PTH is probably most often tested, as it is most reliable.
- PTH levels are affected by a diurnal variation. Levels are highest around 2 AM and lowest around 2 PM. Usually an 8 AM blood specimen is drawn. If the patient works nights, the laboratory should be notified so that changes in the diurnal variation can be factored in.
- Parathryoid panel
- Calcium blood - direct measurement of serum calcium.
- Phosphate - direct measurement of the inorganic phosphate levels in the serum, which are affected by PTH.
- Recent injection of radioisotopes may interfere with this test if radioimmunoassay (RIA) methods are used. However, RIA is not a frequently used method of measuring PTH fragments.
- Drugs that increase PTH include: anticonvulsants, isoniazid, lithium, phosphates, rifampin, and steroids.
- Drugs that decrease PTH include: cimetidine, pindolol, and propranolol.
- Draw fasting morning (8 AM) sample
- Some laboratories require blood in an iced plastic syringe.
- Obtain a serum calcium level determination at the same time if ordered. The serum PTH and serum calcium levels are important for a differential diagnosis.
- Indicate on the laboratory slip the time the blood was drawn, because a diurnal rhythm affects test results.
- Apply pressure or a pressure dressing to the venipuncture site and check the site for bleeding.
- Most PTH specimens are sent to a central laboratory. Transfer on dry ice.
- Freeze immediately
- Store at -20°C and -70°C
- Explain the procedure to the patient.
- Keep the patient on nothing by mouth (NPO) status except for water after midnight on the day of the test.
- Jorde R et al. Population based study on serum ionised calcium, serum parathyroid hormone, and blood pressure. Eur J Endocrinol 1999;141(4):350-7.
- LaGow B et al., eds. PDR Lab Advisor. A Comprehensive Point-of-Care Guide for Over 600 Lab Tests. First ed. Montvale, NJ: Thomson PDR; 2007.
- Pagana K, Pagana TJ eds. Mosby's Manual of Diagnostic and Laboratory Tests. 5th Ed. St. Louis, Missouri. 2014
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