Parathyroid Hormone (PTH) Lab Test, Parathyroid Hormone Assay | Parathyrin, Parathormone (PTH-C-Terminal)

 Parathyroid Hormone (PTH)

Parathormone (PTH), a polypeptide hormone produced in the parathyroid gland, is one of the major factors in the regulation of calcium concentration in extracellular fluid. Three molecular forms of PTH exist: intact ( also called native or glandular hormone), multiple N-terminal fragments, and C-terminal fragments. PTH follows a circadian rhythm pattern; highest values are between 2:am and 4: p.m. and lowest values are at about 8 a.m.

This test studies altered calcium metabolism establishes a diagnosis of hyperparathyroidism and distinguishes nonparathyroid from parathyroid causes of hypercalcemia. A decrease in the level of ionised calcium is the primary stimulus for PTH secretions. Where a rise in calcium inhibits secretions. This normal relation is lost in hyperthyroidism, and PTH will be inappropriately high in relation to calcium. Acute changes in secretory activity are better reflected by the PTH and N-terminal assay. PTH and N-terminal levels are usually decreased when hypercalcemia is due to neoplastic secretions (prostaglandins). PTH and N-terminal levels may be a more reliable indication of secondary hyperparathyroidism in patients with renal failure. Creatinine level is determined concurrently with all PTH assays to determine kidney function and for meaningful interpretation.

Reference Values

Normal

• N-terminal: 8-24 pg/mL or 8-24 ng/L
• Intact molecule: 10-65 pg/mL or 10-65 ng/L
• Calcium: 8.5-10.9 mg/dL (calcium must be tested to properly interpret results)
• C-terminal (biomolecule): 50-330 pg/mL or 60-330 ng/L

Procedure

• Obtain a 10 mL venous blood sample ( EDTA vial) from a patient who has fasted for 10 hours. Collect the sample in chilled vials and keep on ice. Observe standard precautions. Serum or EDTA is used.
• Immediately take specimen to the laboratory and centrifuge at 4∘C after blood clotted.

Clinical Implications

Increased PTH value occur with:

• Primary hyperparathyroidism
• Pseudohyperparathyroidism when there is a primary defect in renal tubular responsiveness to PTH 
• Hereditary vitamin D dependency
• Zollinger-Ellison syndrome  ( increased production of gastrin as a result of tumor in the pancreas)
• Spinal cord injury

Decreased PTH value occur in the following  conditions

• Hypoparathyroidism ( Graves disease)
• Nonparathyroid hypercalcemia
• Secondary hyperparathyroidism (surgical)
• Magnesium deficiency
• Sarcoidosis
• Hyperthyroidism
• DiGeorge's syndrome ( a disorder caused by a defect in chromosome 22 resulting in heart and immune system problems)

Increased PTH-N-terminal value occur in the following conditions.

• Primary hyperparathyroidism
• Secondary hyperparathyroidism ( more reliable than PTH and C-terminal)

Decreased PTH-N-terminal values occur in the following conditions.

• Hypoparathyroidism.
• Nonparathyroid hypercalcemia.
• Aluminum associated osteomalacia.
• Severely impaired bone mineralization.

Increased PTH-C terminal values occur in the following conditions

• Primary hyperparathyroidism. 
• Some neoplasms with elevated calcium.
• Renal failure ( even if parathyroid disease is absent )

Decreased PTH-C- terminal value occur in the following conditions 

• Hypoparathyroidism.
• Nonparathyroid hypercalcemia.

Interfering Factors

• Elevated blood lipids and hemolysis interfere with test methods.
• Milk-alkali syndrome ( Burnett's syndrome, hypercalcemia) may falsely lower PTH levels.
• Recently administered radioisotopes will after results.
• Vitamin D deficiency will increase PTH levels.
• Many drugs alter results; phosphate raise PTH levels up to 125%, and vitamin A and D overdoses decrease PTH levels 
• Lower plasma calcium by 1.5 mg/dL will result in a fourfold increase in PTH levels.

Interventions

Pretest Patient Care

• Explain test purpose and procedure.I|丨
• Remind patient that fasting for at least 10 hours is required. Drow blood by 8 am, because of circadian rhythm changes. Concurrently, also draw blood for testing calcium level.

Post Patient Care

• Have patient resume normal activities.
• Interpret test results and monitor appropriately for calcium imbalance and hypoparathyroidism or hyperparathyroidism.