The intact parathyroid hormone (PTH) test is a diagnostic tool used to assess the function of the parathyroid glands and help understand calcium and phosphorus levels in the blood. This blood test measures the full-length, biologically active form of parathyroid hormone, which plays a central role in mineral balance within the body.
Understanding Parathyroid Hormone
Parathyroid hormone (PTH) is a protein hormone produced and released by the parathyroid glands. These four tiny glands are typically located in the neck, behind the thyroid gland. Their primary function is to regulate the levels of calcium and phosphorus in the bloodstream, ensuring these minerals remain within a narrow, healthy range.
When blood calcium levels drop, the parathyroid glands respond by increasing PTH secretion. PTH then acts on bones, kidneys, and the small intestine to raise calcium levels. It promotes the release of calcium from bones, reduces calcium loss in urine, and indirectly increases calcium absorption from food by stimulating the production of active vitamin D in the kidneys.
The “intact PTH” measured in the test refers specifically to the full-length, 84-amino acid form of the hormone. This is the biologically active form that directly influences calcium and phosphorus metabolism. Measuring intact PTH is important because other inactive fragments of the hormone can exist in the blood, particularly in individuals with kidney problems, and these fragments do not reflect the hormone’s true activity.
Purpose of the Intact PTH Test
Healthcare providers order an intact PTH test for several reasons, primarily to investigate abnormal calcium levels in the blood. The test helps determine if the parathyroid glands are functioning properly in response to these calcium imbalances. It is a key tool in diagnosing conditions where calcium levels are either too high (hypercalcemia) or too low (hypocalcemia).
The test is frequently used to diagnose and monitor parathyroid gland disorders, such as hyperparathyroidism (excess PTH) or hypoparathyroidism (insufficient PTH). It also plays a role in assessing and monitoring individuals with chronic kidney disease, as kidney dysfunction can disrupt the balance of calcium and phosphorus, leading to changes in PTH secretion.
Preparing for and Undergoing the Test
Preparing for an intact PTH test is generally straightforward. In many cases, no special preparation, such as fasting, is required. However, it is always advisable to follow any specific instructions provided by your healthcare provider or the laboratory.
The test involves a standard blood draw, typically performed by a healthcare professional. Blood is usually taken from a vein in your arm, often from the inside of the elbow or the back of the hand. A tourniquet is applied, and the area is cleaned before the needle is inserted to collect the blood sample.
Interpreting Your Intact PTH Test Results
Interpreting intact PTH test results involves considering the measured value in conjunction with blood calcium and vitamin D levels. A typical normal range for intact PTH is approximately 10 to 65 picograms per milliliter (pg/mL), though this can vary slightly between different laboratories. Always refer to the specific reference range provided by the laboratory that conducted your test.
Elevated intact PTH levels with high calcium can suggest primary hyperparathyroidism. In this condition, one or more parathyroid glands produce an excessive amount of PTH, leading to calcium release from bones into the bloodstream. This occurs even when calcium levels are already high, indicating the glands are not responding appropriately to the body’s needs.
High PTH levels with low or normal calcium may indicate secondary hyperparathyroidism. This often arises from conditions causing persistently low blood calcium, prompting the parathyroid glands to overwork to raise calcium levels. Common causes include chronic kidney disease, where kidneys cannot properly excrete phosphorus and activate vitamin D, or severe vitamin D deficiency, which impairs calcium absorption. The elevated PTH is a compensatory response to an underlying issue.
Conversely, suppressed or low intact PTH levels, especially with low calcium, typically point to hypoparathyroidism. This occurs when the parathyroid glands do not produce enough PTH, leading to insufficient calcium in the blood. Causes can include damage to the parathyroid glands during neck or thyroid surgery, autoimmune conditions, or very low magnesium levels.
Low PTH levels alongside high calcium may suggest non-parathyroid causes of hypercalcemia, such as certain cancers or excessive vitamin D intake. In these cases, high calcium is not due to overactive parathyroid glands, and the glands appropriately reduce PTH production in response. It is important to remember that a healthcare professional must interpret these results within the context of an individual’s overall health, symptoms, and other diagnostic findings.