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:
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.
How It Feels
The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.
There is very little chance of a problem from having blood sample taken from a vein.
- You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.
- In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. A warm compress can be used several times a day to treat this.
- Ongoing bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medicines can make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken.
Normal values may vary from lab to lab. Results are usually available in 1 to 2 days.
High PTH levels may be caused by:
- A parathyroid gland growth (hyperplasia) or a parathyroid tumor.
- A low level of calcium in the blood. A low blood calcium level can be caused by kidney disease, kidney failure, severe vitamin D deficiency, or an inability of the intestines to absorb calcium from food.
- Some types of cancer, such as of the lung, kidney, pancreatic, or ovarian cancer.
Low PTH levels may be caused by:
What Affects the Test
Reasons you may not be able to have the test or why the results may not be helpful include:
- Taking medicine that raises PTH levels. These include lithium, furosemide, rifampin, anticonvulsants, thiazide diuretics, and medicines that contain phosphate.
- Taking medicine that lowers PTH levels. These include cimetidine (Tagamet) and propranolol (Betachron ER, Inderal, Inderal LA).
- Being pregnant or breast-feeding.
- Having high cholesterol or triglyceride levels.
- Having a scan that uses a radioactive tracer within 1 week of PTH test.
- Drinking milk or eating milk products right before the test.
What To Think About
- Because PTH can raise calcium levels and lower phosphorus levels, blood tests for calcium and phosphorus are often done at the same time as a test for PTH. For more information, see the medical tests Calcium (Ca) in Blood, Phosphate in Blood, and Total Serum Protein.
- How well your kidneys work can affect how much PTH you have in your blood. For this reason, tests to measure the amount of creatinine in the blood may be done at the same time as a PTH test. For more information, see the medical test Creatinine and Creatinine Clearance.
- A high PTH level along with a high calcium level can cause problems such as osteoporosis, kidney stones, hypertension, kidney failure, peptic ulcer disease, cognitive changes, and problems with the balance of water in the body. About half of all people who have high levels of PTH and calcium in the blood need treatment to correct the abnormal levels. Further testing, such as bone mineral density testing or 24-hour urine calcium testing, may be needed to help make decisions about treatment. For more information, see the medical tests Bone Mineral Density or Calcium (Ca) in Urine.
- An overactive parathyroid gland is often caused by a noncancerous (benign) tumor of the parathyroid gland. Parathyroid tumors tend to grow slowly and may not cause any symptoms for many years. Parathyroid tumors are more common after age 50 and are often found with routine blood tests that are done for other reasons. Treatment includes close observation, medicines, or surgery to remove the tumor.
Other Works Consulted
- Pagana KD, Pagana TJ (2006). Mosby’s Manual of Diagnostic and Laboratory Tests, 3rd ed. St. Louis: Mosby.
|Author||Bets Davis, MFA|
|Associate Editor||Tracy Landauer|
|Primary Medical Reviewer||Caroline S. Rhoads, MD - Internal Medicine|
|Specialist Medical Reviewer||Matthew I. Kim, MD - Endocrinology & Metabolism|
|Last Updated||June 11, 2008|