The Intact Parathyroid Hormone (PTH) test is a blood analysis that measures the level of active PTH circulating in the bloodstream. This measurement helps doctors assess the function of the four small parathyroid glands, located in the neck near the thyroid. The test focuses on the biologically active form of the hormone, providing insight into how the body manages its mineral balance. Results are generally used to help determine the cause of abnormal calcium levels.
The Role of Parathyroid Hormone in the Body
Parathyroid Hormone (PTH) is a peptide hormone that regulates calcium and phosphorus levels. The parathyroid glands constantly monitor blood calcium concentration and secrete PTH in response to any drop in those levels. This negative feedback loop maintains calcium within a healthy range.
Once released, PTH acts on three main organs to restore calcium balance. It signals the bones to release stored calcium into the bloodstream via bone resorption. Simultaneously, PTH instructs the kidneys to reabsorb more calcium back into the blood and excrete excess phosphorus through the urine.
PTH also stimulates the kidneys to convert inactive Vitamin D into its active form, calcitriol. This activated Vitamin D travels to the small intestine, enhancing the absorption of calcium from food. These coordinated actions ensure that the body’s nerves, muscles, and heart function correctly.
Understanding the Intact PTH Measurement
Parathyroid hormone is synthesized as a chain of 84 amino acids, known as PTH (1-84), which is the full, biologically active molecule. Once released, this active hormone is rapidly broken down into multiple, physiologically inactive fragments. Since the active PTH (1-84) has a half-life of only a few minutes, measuring all fragments together would give a misleading picture of current gland activity.
The “intact PTH” test was developed to overcome this challenge by only measuring the full 84-amino-acid chain. This is achieved using an immunoassay that employs two different antibodies. One antibody binds to the N-terminal end of the hormone, and the second binds to the C-terminal end.
Only the full, intact PTH molecule can connect with both antibodies simultaneously, creating a “sandwich” complex that is measured. This two-site detection system provides the most accurate assessment of the hormone performing regulatory work. The test is performed through a standard blood draw.
Clinical Reasons for Ordering the Test
A healthcare provider typically orders the Intact PTH test when a patient’s blood calcium level is outside the normal range. The test’s main purpose is to determine if the parathyroid glands are the cause of the calcium abnormality. This helps distinguish between parathyroid-mediated disorders and other possible causes, such as certain cancers or nutritional deficiencies.
The test is frequently ordered to diagnose hyperparathyroidism, a condition where the glands produce too much hormone. Primary hyperparathyroidism occurs when a problem with the gland itself, often a benign tumor (adenoma), causes excessive PTH secretion and high calcium levels. Conversely, the test also diagnoses hypoparathyroidism, where low PTH production results in low blood calcium.
The test is also a standard tool for monitoring patients with chronic kidney disease (CKD). In CKD, the kidneys cannot efficiently convert Vitamin D or excrete phosphorus, leading to low calcium and high phosphorus levels. The body compensates by continuously producing high amounts of PTH, known as secondary hyperparathyroidism, which can cause bone problems. For patients on dialysis, PTH levels are routinely monitored to manage this bone and mineral disorder.
Interpreting the Test Results
Interpreting the Intact PTH result is rarely done in isolation and must be viewed alongside simultaneous measurements of blood calcium and Vitamin D levels. The relationship between PTH and calcium is inverse: if calcium is low, PTH should be high, and vice versa. Disruptions in this expected pattern point toward a parathyroid disorder.
A high PTH level combined with a high calcium level is the classic signature of primary hyperparathyroidism. The parathyroid glands are overactive, inappropriately driving calcium levels up. If PTH is high but calcium is low, this suggests secondary hyperparathyroidism, where the glands are responding appropriately to a problem elsewhere, such as Vitamin D deficiency or chronic kidney failure.
A low PTH level paired with a low calcium level suggests hypoparathyroidism, indicating the glands are not producing enough hormone. This often results from damage to the glands following neck surgery. Conversely, a low PTH level with high calcium suggests the high calcium is caused by something other than the parathyroid glands, such as malignancies or excessive Vitamin D intake.
Normal reference ranges for intact PTH vary slightly between laboratories, often falling within 15 to 65 picograms per milliliter (pg/mL). However, a PTH level within the normal range may still be considered inappropriate if the calcium level is abnormal. For instance, a “normal” PTH level when calcium is high signals that the glands are not shutting down, which can confirm a diagnosis of primary hyperparathyroidism.