Type of test Blood

Normal findings

 Intact (whole): 10-65 pg/mL or 10-65 ng/L (SI units)

N terminal: 8-24 pg/mL

C terminal: 50-330 pg/mL

Test explanation and related physiology

This test is useful in establishing a diagnosis of hyperparathyroidism and distinguishing nonparathyroid from parathyroid causes of hypercalcemia. Increased PTH levels are seen in patients with hyperparathyroidism (primary, secondary, or tertiary); in patients with nonparathyroid, ectopic PTH-producing tumors (pseudohyperparathyroidism); or as a normal compensatory response to hypocalcemia in patients with malabsorption or vitamin D deficiency.

It is important to measure serum calcium simultaneous with the measurement of PTH. Most laboratories have a PTH/calcium nomogram already made up, indicating what PTH level is considered normal for each calcium level.

Decreased PTH levels are seen in patients with hypoparathyroidism or as a compensatory response to hypercalcemia in patients with metastatic bone tumors, sarcoidosis, vitamin D intoxication, or milk-alkali syndrome. Of course, surgical ablation of the parathyroids is another cause of hypoparathyroidism.

Interfering factors

* Drugs that increase PTH include anticonvulsants, isoniazid, lithium, rifampin, and steroids

* Drugs that decrease PTH include cimetidine and propranolol

Procedure and patient care

 • See inside front cover for Routine Blood Testing.

• Fasting: yes

• Blood tube commonly used: red

• Obtain a morning blood specimen, because diurnal rhythm affects PTH levels. (Check with the laboratory if the patient works at night.)

 • Note that some laboratories require blood in an iced plastic syringe.

• Obtain a serum calcium level determination at the same time if ordered. Serum PTH and serum calcium levels are important in the differential diagnosis.

 • Indicate the time the blood was drawn on the laboratory slip because a diurnal rhythm affects test results.

Abnormal findings

 Increased levels

- Hyperparathyroidism secondary to adenoma or carcinoma of the parathyroid gland

- Non–PTH-producing tumors (paraneoplastic syndrome)

- Lung carcinoma

- Kidney carcinoma

- Hypocalcemia

- Chronic renal failure

- Malabsorption syndrome

- Vitamin D deficiency

- Rickets

- Osteomalacia

- Congenital renal defect

Decreased levels

- Hypoparathyroidism

- Hypercalcemia

- Metastatic bone tumor

- Sarcoidosis

- Autoimmune destruction of the parathyroid glands

- Vitamin D intoxication

- Milk-alkali syndrome

- Graves disease

- Hypomagnesemia

- DiGeorge syndrome

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