Parathyroid hormone assay, parathyrin, parathormone, PTH-C-Terminal
This test measures a substance called parathyroid hormone (PTH) in your blood. PTH is made by four tiny parathyroid glands in your neck. PTH circulates in your blood and is needed to regulate the level of calcium in your blood. Your heart, bones, nervous system, and kidneys need a normal calcium level in the blood to work the way they should.
If your calcium level is too low, your parathyroid glands release PTH to get more calcium into your blood. If your calcium level is too high, your parathyroid glands stop producing PTH. Measuring the amount of PTH in your blood helps your doctor find out the cause of an abnormal calcium level.
This test is usually done early in the morning, the best time best to measure PTH.
You may need this test if an earlier blood test found that your calcium level was too high or too low. You may also need this test if you have signs or symptoms of an abnormal calcium level. PTH levels help your doctor find out whether an abnormal calcium level is from a problem with your parathyroid glands or has another cause.
Signs and symptoms of high calcium, or hypercalcemia, include:
Aches and pains
Symptoms of low calcium, or hypocalcemia, include:
Tingling and numbness
Depression or other mood changes
Muscle spasms or seizures
Your doctor may order other blood tests along with a PTH test. It's common to have your blood calcium level checked along with the PTH level. Other blood mineral levels may also be checked. You may also have tests to measure your kidney function.
Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.
Results are given in picograms per milliliter (pg/mL). This test measures three forms of PTH: intact molecule, N-terminal fragments, and C-terminal fragments. Normal levels are:
N-terminal: 8 to 24 pg/mL
Intact molecule: 10 to 65 pg/mL
C-terminal: 50 to 330 pg/mL
Some of the more common causes of high PTH levels are:
Overactive parathyroid glands, a condition called hyperparathyroidism; this can have different causes
Failure of the kidneys to respond normally to PTH
Inherited vitamin D deficiency
Spinal cord injury
Low calcium not related to the parathyroid glands
Some of the more common causes of low PTH levels are:
Underactive parathyroid glands, a condition called hypoparathyroidism; this can have different causes
Overactive thyroid gland, or hyperthyroidism
Elevated calcium not related to the parathyroid glands
Your doctor will look at the PTH results along with the blood calcium level and possibly other tests to better understand what the results mean.
The test requires a blood sample, which is drawn through a needle from a vein in your arm.
Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore.
Conditions such as higher than normal blood fats, vitamin D deficiency, and milk-alkali syndrome, also called Burnett's syndrome, can interfere with your test results. Many types of medications can also alter the results, including drugs containing phosphates and vitamin A or D overdoses.
You may need to fast for 10 hours before the test. Be sure your doctor knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illicit drugs you may use.