What Is a Parathyroid Hormone Blood Test?

You probably already know you need calcium for strong bones and teeth. But did you know that you need normal levels of this important mineral for healthy blood vessels, muscles, and nerves, too?

If you recently had a blood test that showed very high or very low calcium levels, your doctor may suggest you get another type of blood test.

This test would measure your parathyroid hormone (PTH) levels.

PTH is made by four tiny parathyroid glands in your neck. These glands control calcium levels in your blood. When calcium levels are too low, the glands release PTH to bring the calcium levels back up into a normal range. When your calcium levels rise, the glands stop releasing PTH.

Measuring PTH can help explain the reason for abnormal calcium levels.

A parathyroid hormone blood test is sometimes called a parathyroid hormone assay or a parathyrin test.

Why Would Your Doctor Order It?

A PTH blood test can help your doctor figure out whether your abnormal calcium levels are caused by your parathyroid glands. If your test shows that your PTH level is appropriate for your calcium level, then there is some other cause of high or low calcium levels.

Your doctor may recommend this blood test if you’re showing symptoms of hypercalcemia (too much calcium in your blood) or hypocalcemia (too little calcium in your blood).

Hypercalcemia symptoms include:

Hypocalcemia symptoms include:

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Risks and Benefits

The risks of taking a PTH blood test are minor. You may feel some pain where the needle goes into your skin. That area may be a little sore afterward.

As with any blood test, there is a very slight chance of infection or bruising. Some people feel a little lightheaded after a blood test.

The most important benefit of a PTH blood test is that it lets your doctor know whether your body might be making too much or too little parathyroid hormone.

If it appears as though that’s the problem, then more tests can be done to officially diagnose your condition. Just as important, the test can also rule out parathyroid disease.

This will allow you and your doctor to look for other causes of your abnormal calcium levels.

Preparing for the Test

Usually you have to stop eating 10 hours before the blood test. You may need to stop taking certain medications or supplements the day before or the day of the test.

But check with your doctor first before you stop taking prescription medications, as well as over-the-counter medicines and supplements.

During and After the Test

Blood is taken from a vein in your arm.

The person drawing blood may tie a rubber band around your upper arm first. This, along with making a fist, can help make the veins more visible near the surface of the skin. The needle that enters the vein is attached to a small test tube.

Only a little blood is needed for a PTH test. Once enough blood has been drawn, the technician will remove the needle and put a bandage on you.

The blood sample is then sent to a lab where it will be measured for PTH, calcium, and possibly hormones, minerals, or other substances.

Results

Three forms of PTH are measured in this test. The exact normal ranges vary based on the lab doing the testing. The results are described in picograms per milliliter (pg/mL). Discuss the results with your health care provider so that you understand what these numbers mean. The three forms of PTH and some fairly typical normal ranges are:

  • N-terminal: 8 to 24 pg/mL
  • C-terminal: 50 to 330 pg/mL
  • Intact molecule: 10 to 65 pg/mL

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It could take up to a few days to get your test results back, depending on the lab. But it could be sooner, especially if your doctor wants a faster turnaround.

High PTH levels could be caused by overactive parathyroid glands. This is called hyperparathyroidism. However, there are other potential causes of high PTH levels, such as:

  • Inherited low vitamin D level
  • Spinal cord injury
  • Low calcium unrelated to parathyroid glands
  • Kidneys that don’t respond normally to PTH

Low PTH levels could be related to underactive parathyroid glands (hypoparathyroidism). Other causes could include:

Will I Need Other Tests?

Depending on your PTH levels, you may need more tests.

For example, if your PTH levels are high, but your calcium levels are still low, it could be that your parathyroid glands are working properly. Your doctor may then test your vitamin D, phosphorous, and magnesium levels to see whether they are affecting your calcium levels.

On the other hand, if your calcium levels are high and your PTH levels are also above normal, you may have hyperparathyroidism. Your doctor may order an X-ray or other imaging tests to examine your parathyroid glands.

Hyperparathyroidism may be treated with surgery to remove the glands if they are enlarged or contain a tumor.

If your condition is mild and you have no symptoms, such as kidney problems or weakened bones, your doctor may suggest that your calcium and PTH levels be checked regularly. No surgery may be necessary, at least for a while.

WebMD Medical Reference Reviewed by Minesh Khatri, MD on January 18, 2017

Sources

SOURCES:

National Institutes of Health: “Calcium.”

American Association of Endocrine Surgeons: “Parathyroid glands: Function.”

Lab Tests Online: “PTH.”

University of Rochester Medical Center: “Parathyroid hormone.”

National Heart, Lung, and Blood Institute: “What to expect with blood tests.”

Mayo Clinic: “Hyperparathyroidism: Treatment and drugs.”

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