Definition

The parathyroid hormone (PTH) test is a blood test performed to determine the serum levels of a hormone secreted by the parathyroid gland in response to low blood calcium levels. PTH works together with vitamin D to maintain healthy bones. The parathyroid glands are small paired glands located near the thyroid gland at the base of the neck.

  Purpose

The PTH level is measured to evaluate the level of blood calcium. It is routinely monitored in patients with a kidney disorder called chronic renal failure (CRF). Because PTH is one of the major factors affecting calcium metabolism, the PTH test helps to distinguish nonparathyroid from parathyroid causes of too much calcium in the blood (hypercalcemia).

Differential diagnosis of hyperparathyroidism

PTH is also useful in the differential diagnosis of overactive parathyroid glands (hyperparathyroidism). Primary hyperparathyroidism is most often caused by a benign tumor in one or more of the parathyroid glands. It is rarely caused by parathyroid cancer. Patients with this condition have high PTH and calcium levels.

Secondary hyperparathyroidism is often seen in patients with chronic renal failure (CRF). The kidneys fail to excrete sufficient phosphate, and the parathyroid gland secretes PTH in an effort to lower calcium levels to balance the calcium-phosphate ratio. Because of the constant stimulation of the parathyroid, CRF patients have high PTH and normal or slightly low calcium levels.

Tertiary hyperparathyroidism occurs when CRF causes a severe imbalance in the calcium-phosphate ratio, leading to very high PTH production that results in hypercalcemia. Patients with this condition have high PTH and high calcium levels.

Specific PTH assays

PTH is broken down in the body into three different molecular forms: the intact PTH molecule and several smaller fragments which include an amino acid or N-terminal, a midregion or midmolecule, and a carboxyl or C-terminal. Two tests are currently used to measure intact PTH and its terminal fragments. While both tests are used to diagnose hyper-or hypoparathyroidism, each test also has specific applications as well. The C-terminal PTH assay is used to diagnose the ongoing disturbances in PTH metabolism that occur with secondary and tertiary hyperparathyroidism. The assay for intact PTH and the N-terminal fragment, which are both measured at the same time, is more accurate in detecting sudden changes in the PTH level. For this reason, the N-terminal PTH assay is used to monitor a patient's response to therapy.

  Precautions

Drug interactions

Some prescription drugs affect the results of PTH tests. Drugs that increase PTH levels include phosphates, anticonvulsants, steroids, isoniazid, lithium, and rifampin. Drugs that decrease PTH include cimetidine and propranolol.

Timing

PTH levels are subject to daily variation, ranging from a peak around 2:00 A.M. to a low point around 2:00 P.M. Specimens are usually drawn at 8:00 A.M. The laboratory should be notified if the patient works a night shift so that this difference in biological rhythm can be taken into account.

Other serum level tests

Due to the relationship between PTH and calcium, calcium levels should be tested at the same time as PTH. Most laboratories have established reference values to indicate what PTH level is normal for a particular calcium level. In addition, the effects of PTH on kidney function and bone strength indicate that serum calcium, phosphorus, and creatinine levels should be measured together with PTH. The creatinine test measures kidney function and aids in the diagnosis of parathyroid dysfunction.

  Description

The PTH test is performed on a sample of the patient's blood, withdrawn from a vein into a vacuum tube. The procedure, which is called a venipuncture, takes about five minutes.

  Preparation

The patient should have nothing to eat or drink from midnight of the day of the test.

  Risks

Risks for this test are minimal, but may include slight bleeding from the puncture site, a small bruise or swelling in the area, or fainting or feeling lightheaded.

Normal results

Reference ranges for PTH tests vary somewhat depending on the laboratory, and must be interpreted in association with calcium results. The following ranges are typical:

• Intact PTH: 10-65 pg/mL
• PTH N-terminal (includes intact PTH): 8-24 pg/mL
• PTH C-terminal (includes C-terminal, intact PTH, and midmolecule): 50-330 pg/mL.

Abnormal results

When measured with serum calcium levels, abnormally high PTH values may indicate primary, secondary, or tertiary hyperparathyroidism, chronic renal failure, malabsorption syndrome, and vitamin D deficiency. Abnormally low PTH levels may indicate hypoparathyroidism, hypercalcemia, and certain malignancies.

   Key Terms:

Assay An analysis of the chemical composition or strength of a substance. Hypercalcemia Abnormally high levels of blood calcium. Hyperparathyroidism Overactivity of the parathyroid glands. Symptoms include generalized aches and pains, depression, and abdominal pain. Hypoparathyroidism Insufficient production of parathyroid hormone, which results in low levels of blood calcium.

   Resources:

Books

• Jacobs, David S., et al. Laboratory Test Handbook. 4th ed. New York: Lexi-Comp Inc., 1996.
• Pagana, Kathleen Deska. Mosby's Manual of Diagnostic and Laboratory Tests. St. Louis: Mosby, Inc., 1998.
• Cahill, Mathew. Handbook of Diagnostic Tests. Springhouse, PA: Springhouse Corporation, 1995.