A PTH (parathyroid hormone) test measures the level of PTH in the blood. PTH is a hormone made by the parathyroid glands, which are four pea-sized glands in the neck. PTH controls the level of calcium in the blood, which is essential for the proper functioning of nerves, muscles, and the heart, as well as for keeping bones and teeth healthy and strong.

A PTH test is most often used along with calcium testing to diagnose hyperparathyroidism, a condition in which the parathyroid glands produce too much PTH, or hypoparathyroidism, a condition in which the parathyroid glands produce too little PTH. PTH levels that are too high or too low can cause serious health problems.

If a PTH test shows a higher than normal level of PTH, it may indicate hyperparathyroidism, a benign tumor of the parathyroid gland, kidney disease, a vitamin D deficiency, or a disorder that makes it difficult to absorb calcium from food. If a PTH test shows a lower than normal level of PTH, it may indicate hypoparathyroidism or an overdose of vitamin D or calcium.

Normal ranges for PTH levels can vary from lab to lab, but in general, the normal range for the PTH blood test known as "PTH, intact" is 15 to 65 picograms per milliliter (pg/mL) . A PTH test can also show whether there is too little or too much calcium in the blood, indicating a thyroid issue.

A PTH test is a useful tool in the differential diagnosis and management of hypercalcemia, and it can help diagnose tumors and hyperplasia of the parathyroid gland, as well as localize hyperfunctioning parathyroid tissue by assay of samples obtained via venous catheterization.