Parathyroid hormone (PTH) is secreted by four tiny glands located in the neck, near or attached to the back of the thyroid gland. The intact PTH blood test measures the concentration of this hormone in the blood. This test specifically measures the active, full-length form of the hormone, which indicates how well the parathyroid glands are functioning. PTH is the body’s primary regulator of calcium levels, and the test helps doctors evaluate disorders of calcium balance.
How Parathyroid Hormone Regulates the Body
Parathyroid hormone maintains calcium and phosphate concentrations in the blood. This regulation uses a precise feedback loop: when serum calcium drops, the parathyroid glands increase PTH secretion. Conversely, an increase in serum calcium suppresses the release of PTH.
PTH exerts its influence on three main target organs to raise blood calcium levels. In the skeleton, PTH triggers the release of calcium and phosphate into the bloodstream through a process called bone resorption. Simultaneously, the hormone acts on the kidneys to enhance the reabsorption of calcium back into the blood while promoting the excretion of phosphate in the urine.
The hormone’s effect on the kidneys also involves stimulating the production of the most active form of Vitamin D, known as calcitriol. Calcitriol then travels to the small intestine, where it significantly increases the absorption of dietary calcium. This coordinated action ensures that calcium levels are quickly restored, which is necessary for proper nerve signaling, muscle function, and bone health.
The Specifics of the Intact PTH Test
The parathyroid hormone is initially synthesized as a full-length molecule composed of 84 amino acids, designated as PTH 1-84. This PTH 1-84 molecule is the biologically active form. Once secreted, the hormone has a very short half-life, measured in minutes, and rapidly breaks down into smaller, inactive fragments.
Older laboratory methods could not distinguish effectively between the active hormone and these inactive fragments, which often accumulated, particularly in patients with kidney dysfunction. The intact PTH test uses specialized antibodies to measure only the complete, biologically active PTH 1-84 molecule. This focused measurement provides a much more accurate snapshot of the parathyroid gland’s current secretory activity.
The test is performed via a simple blood draw, typically from a vein in the arm. Although fasting is usually not required, the sample should be taken at a consistent time of day, as PTH levels naturally fluctuate. Because interpretation depends on mineral balance, a blood calcium measurement is nearly always ordered simultaneously.
Conditions Monitored by the Test
The intact PTH test is a primary diagnostic tool used when a patient exhibits abnormal calcium levels, either high (hypercalcemia) or low (hypocalcemia). It helps physicians differentiate between conditions where the parathyroid gland is the cause of the imbalance versus those where the problem lies elsewhere.
Primary Hyperparathyroidism (PHPT)
PHPT is characterized by the parathyroid gland producing too much PTH independently of the body’s needs, often due to a benign tumor called an adenoma. This inappropriate over-secretion of PTH leads directly to elevated serum calcium levels. The PTH test confirms this diagnosis when both PTH and calcium levels are high, or when PTH is inappropriately high-normal despite high calcium.
Secondary Hyperparathyroidism
A major application of the test is in monitoring patients with Chronic Kidney Disease (CKD). As kidney function declines, the body’s ability to excrete phosphate and activate Vitamin D is impaired, leading to low calcium and high phosphate. This low calcium constantly stimulates the parathyroid glands to produce more PTH in a compensatory effort, a condition known as secondary hyperparathyroidism.
If secondary hyperparathyroidism is left untreated over a long period, the continuously stimulated parathyroid glands can become permanently overactive and function autonomously. This progression results in tertiary hyperparathyroidism, where PTH and calcium levels can become elevated even after the underlying cause, such as kidney failure, has been addressed. The intact PTH test is also used to diagnose hypoparathyroidism, a condition where the glands produce insufficient PTH, resulting in low blood calcium.
Understanding Your PTH Test Results
Interpreting the results of an intact PTH test requires considering the simultaneous measurements of calcium and sometimes Vitamin D. Reference ranges for PTH can vary between laboratories, but a typical normal range for the intact assay is 10 to 55 picograms per milliliter (pg/mL). The interpretation involves assessing the relationship between the PTH level and the corresponding serum calcium level.
A combination of high PTH and high calcium suggests primary hyperparathyroidism, indicating that the gland is overproducing the hormone. If a patient presents with a high PTH but a low calcium level, this points toward secondary hyperparathyroidism. This scenario often occurs due to a lack of Vitamin D or poor kidney function, where the parathyroid glands are appropriately reacting to low calcium to correct the imbalance.
Conversely, low PTH combined with low calcium suggests hypoparathyroidism, meaning the glands are failing to secrete enough of the hormone to maintain proper calcium levels. If a patient has an abnormal calcium level but a PTH level that is normal or suppressed, it suggests a cause independent of the parathyroid glands, such as malignancy or Vitamin D intoxication. Physicians rely on the trend of PTH and calcium levels over time, rather than a single value, to guide diagnosis and treatment, particularly in complex cases like chronic kidney disease.