The Parathyroid Hormone (PTH) test is a blood analysis that measures the amount of PTH circulating in the bloodstream. PTH is a small protein produced by the four tiny parathyroid glands, which are located in the neck, typically situated just behind the thyroid gland. The test is used to investigate the cause of abnormal calcium levels in the blood, helping to distinguish between problems originating in the parathyroid glands and those caused by other health issues.
The Biological Role of Parathyroid Hormone
Parathyroid hormone acts as the primary regulator of calcium concentration in the blood, ensuring this mineral remains within a stable range. The parathyroid glands secrete PTH in direct response to a slight drop in calcium levels, operating a precise feedback loop. Stable calcium is required for nerve signaling, muscle contraction, and heart function. PTH achieves its goal by targeting three main areas: the bones, the kidneys, and the small intestine. In the bones, the hormone indirectly stimulates specialized cells to dissolve bone tissue (resorption), which releases stored calcium into the bloodstream.
In the kidneys, PTH encourages the reabsorption of calcium, minimizing loss through urine. Simultaneously, the hormone causes the kidneys to excrete more phosphate. Furthermore, PTH stimulates the production of the active form of Vitamin D within the kidneys. This active Vitamin D then acts on the small intestine to increase the efficiency of dietary calcium absorption.
Preparing for and Performing the PTH Test
A healthcare provider often orders a PTH test when a patient exhibits persistently abnormal blood calcium levels or presents with symptoms suggestive of an imbalance. These symptoms can range from increased thirst and frequent urination (associated with high calcium) to muscle cramps and tingling (which may occur with low calcium). The test is also used to monitor individuals with chronic kidney disease or those with unexplained bone loss.
Preparation for the blood draw may vary, but many laboratories require fasting for 10 to 12 hours before the sample is taken. PTH levels fluctuate naturally throughout the day, so the test is often timed for the morning to ensure consistency in measurement. The procedure itself is a standard venipuncture, where a small amount of blood is collected from a vein in the arm.
The blood sample is sent to a laboratory to measure the concentration of the intact PTH molecule. Because PTH concentration changes rapidly in response to calcium levels, a blood calcium test is almost always performed on the same sample for accurate context. The results are interpreted together to understand the patient’s mineral regulation status.
Interpreting High and Low PTH Levels
The normal range for PTH is approximately 10 to 55 picograms per milliliter (pg/mL), though this range can vary slightly depending on the specific laboratory performing the analysis. Crucially, a PTH result must be evaluated alongside the measured calcium level. A PTH value that falls within the “normal” range may still be considered inappropriate if the blood calcium is high.
An elevated PTH level, a condition known as hyperparathyroidism, can be categorized into two main types based on the accompanying calcium level. The most common finding is primary hyperparathyroidism, which is defined by high PTH levels occurring concurrently with high calcium levels. This situation typically means the parathyroid glands themselves are overactive, often due to a benign tumor called an adenoma, which causes them to release too much hormone regardless of the blood’s calcium concentration.
The second type is secondary hyperparathyroidism, where the PTH is high but the calcium level is normal or low. This is a physiological response where the glands are reacting to a persistent underlying issue causing low calcium. Long-term kidney disease is a frequent cause, as damaged kidneys cannot properly activate Vitamin D or excrete phosphate, leading to chronic low calcium that forces the parathyroid glands to overproduce PTH. Vitamin D deficiency or poor calcium absorption can also lead to this high PTH state.
Conversely, a low PTH level, called hypoparathyroidism, is most commonly seen after neck surgery where the parathyroid glands may have been damaged or removed. The low PTH is usually accompanied by a low blood calcium level, as the body cannot mobilize calcium from the bones or efficiently absorb it from the gut.
The presence of high calcium and low or undetectable PTH suggests the high calcium is caused by something other than the parathyroid glands, such as certain cancers or excessive Vitamin D intake. The PTH test provides essential diagnostic information that guides healthcare providers toward the correct diagnosis and treatment plan.