A parathyroid hormone (PTH) blood test measures the level of parathyroid hormone in the blood. This test is used to help identify hyperparathyroidism or to find the cause of abnormal calcium levels. PTH controls calcium and phosphorus levels in the blood.

PTH is made by the parathyroid glands, which are four pea-sized glands that lie behind the thyroid gland. If the blood calcium level is too low, the parathyroid glands release more PTH. This causes the bones to release more calcium into the blood and reduces the amount of calcium released by the kidneys into the urine. Also, vitamin D is converted to a more active form, causing the intestines to absorb more calcium and phosphorus. If the calcium level is too high, the parathyroid glands release less PTH, and the whole process is reversed.

PTH levels that are too high or too low can cause problems with the kidneys and bones and cause changes in calcium and vitamin D levels.

Tests for calcium and phosphorus levels in the blood may be done at the same time as a PTH test.

Why It Is Done

A test for parathyroid hormone (PTH) is done to:

• Help identify hyperparathyroidism.
• Find the cause of an abnormal blood calcium level.
• Check to see whether a problem with the parathyroid glands is causing the abnormal calcium level.
• Watch for problems in people who have chronic kidney disease.

How To Prepare

Do not eat or drink anything except water for 8 to 10 hours before the test. A PTH test is usually done before 10 a.m. because PTH levels go up quickly after you wake up. But your health professional will discuss any changes to this schedule with you. Tell your doctor if you work nights so that you can arrange to have your blood tested shortly after you wake up.

Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results may mean. To help you understand the importance of this test, fill out the medical test information form (What is a PDF document?) .

How It Is Done

The health professional drawing blood will:

• Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
• Clean the needle site with alcohol.
• Put the needle into the vein. More than one needle stick may be needed.
• Attach a tube to the needle to fill it with blood.
• Remove the band from your arm when enough blood is collected.
• Put a gauze pad or cotton ball over the needle site as the needle is removed.
• Put pressure to the site and then put on a bandage.