The parathyroid glands, located in the neck near the thyroid, play a role in regulating the body’s calcium balance. These glands produce parathyroid hormone (PTH), which maintains stable levels of calcium and phosphorus in the bloodstream. A “PTH intact” blood test measures this hormone to assess parathyroid gland function and investigate conditions related to mineral metabolism.
Understanding Parathyroid Hormone
Parathyroid hormone is synthesized and secreted by the parathyroid glands. Its primary function involves regulating calcium concentrations in the blood, working in conjunction with vitamin D and calcitonin. PTH directly influences calcium levels by promoting its release from bones, increasing its reabsorption in the kidneys, and indirectly enhancing calcium absorption from the intestines through its effect on vitamin D activation. Concurrently, PTH also impacts phosphorus levels by increasing its excretion through the kidneys.
The “intact PTH” measurement refers to the full-length, biologically active form of the hormone, which consists of 84 amino acids. This form is the most accurate indicator of parathyroid gland activity because it is the only form that can bind to and activate PTH receptors in target tissues. Measuring intact PTH provides a precise assessment of parathyroid function compared to measuring fragments of the hormone.
Why the Test is Performed
Healthcare providers order an intact PTH blood test for symptoms or conditions suggesting an imbalance in calcium or phosphorus levels. These include unexplained fatigue, muscle weakness, bone pain, or kidney stones. The test is often performed when routine blood work reveals abnormal calcium levels, either too high (hypercalcemia) or too low (hypocalcemia), as PTH is a primary regulator of this mineral.
The intact PTH test is also used to diagnose and monitor parathyroid disorders. It helps distinguish between different causes of hypercalcemia, such as primary hyperparathyroidism (where glands are overactive) or other non-parathyroid causes. For individuals with chronic kidney disease, monitoring PTH levels is important because impaired kidney function disrupts calcium and phosphorus balance, leading to secondary hyperparathyroidism. It also aids in evaluating conditions like osteoporosis when an underlying parathyroid issue is suspected.
Interpreting Test Results
Interpreting intact PTH test results requires careful consideration of accompanying calcium and phosphorus levels, as these minerals are intricately linked. A typical normal range for intact PTH is generally between 10 to 65 picograms per milliliter (pg/mL), though this can vary between laboratories. Values within this range, combined with normal calcium and phosphorus, indicate healthy parathyroid gland function.
Elevated PTH levels, especially when coupled with high calcium, indicate primary hyperparathyroidism, where one or more parathyroid glands produce excessive hormone independently. High PTH with low or normal calcium and high phosphorus suggests secondary hyperparathyroidism, commonly seen in chronic kidney disease, where the glands overproduce PTH in response to low calcium or high phosphorus. Very high PTH levels, sometimes with elevated calcium, in long-standing secondary hyperparathyroidism indicate tertiary hyperparathyroidism, where the glands become autonomous.
Conversely, low intact PTH levels, particularly with low calcium and high phosphorus, indicate hypoparathyroidism. This condition can result from accidental damage to the parathyroid glands during neck surgery, such as thyroidectomy, or from autoimmune disorders where the body’s immune system attacks the glands. Very low PTH may also be associated with high calcium, which could suggest non-parathyroid causes of hypercalcemia, such as certain cancers.
The Testing Process
An intact PTH blood test is a straightforward procedure, similar to other routine blood draws. Patients may need to fast for several hours before the test, often overnight. Inform your healthcare provider about all medications, supplements, and vitamins, as some can influence PTH levels. Specific instructions regarding medication adjustments will be provided if necessary.
During the test, a healthcare professional will draw a blood sample. The procedure involves cleaning the skin, applying a tourniquet, and inserting a small needle to collect blood into a tube. The process is quick and well-tolerated. The time of day the sample is collected can influence results, as PTH levels vary throughout the day, often highest in the morning.