Parathyroid hormone (PTH), also known as parathormone, is a hormone released by the parathyroid glands. The four parathyroid glands are located in your neck, next to the thyroid gland. These glands are responsible for managing calcium, vitamin D, and phosphorus levels in the blood and bones. PTH helps regulate calcium levels in the blood. Thus, calcium imbalances in the blood may be a sign of parathyroid gland or PTH issues.
Calcium levels in the blood signal the parathyroid glands to release PTH. When calcium levels are low, the parathyroid glands increase PTH production. When the calcium levels are high, the glands slow secretion of PTH. Sometimes doctors need to measure how much PTH is in your blood. Because of the relationship between calcium and PTH in the blood, often both are tested at the same time.
Healthy calcium levels are essential for our bodies to function properly. PTH may need to be measured:
- if you are having symptoms of too much calcium in the blood (fatigue, nausea, thirst, abdominal pain)
- if you are having symptoms of too little calcium in the blood (abdominal pain, muscle cramps, tingling fingers)
- if your blood calcium test comes back abnormal
- to figure out the cause of too much or too little calcium in the blood
Too much calcium could be a sign of hyperparathyroidism—a condition caused by overactive parathyroid glands, i.e., glands that produce too much PTH. Excess calcium in the blood can lead to kidney stones, osteoporosis, and osteomalacia (a bone-weakening disease like osteoporosis).
Too little calcium could be a sign of hypoparathyroidism—a condition caused by underactive parathyroid glands, i.e., glands that are not producing enough PTH. Too little calcium in the blood could lead to muscle spasms, heart rhythm disturbances, and tetany (overstimulated nerves).
Doctors may also order this test to:
- check parathyroid functioning
- distinguish between parathyroid-related and non-parathyroid-related disorders
- monitor the effectiveness of treatment in parathyroid-related issues
- determine the cause of low phosphorus levels in the blood
- determine why a patient’s severe osteoporosis is not responding to treatment
- monitor chronic conditions, such as kidney disease
Most commonly, your doctor will recommend a PTH measurement if you have signs of too much or too little calcium in your blood, before having hyperparathyroidism surgery, or to assess the functioning of your parathyroid gland.
Before the Test
PTH is measured through a blood draw. The process of taking a blood sample for testing is called venipuncture. You may need to fast, or refrain from eating or drinking, for a specific period of time before the blood test. Ask your doctor about any specific pretest considerations.
Your doctor will take blood from a vein, usually from the inner elbow or back of the hand. First, your doctor will sterilize the area with an antiseptic. Then, he or she will wrap a plastic band around your arm to apply pressure and to help the veins swell with blood.
Once the veins are swollen, your doctor will insert a sterile needle directly into the vein. The blood will collect into an attached vial. Once enough blood has been collected for the sample, the plastic band is untied and the needle is removed from the vein. The site of the needle insertion will be cleaned and bandaged, if necessary.
Some individuals experience only slight pain from the needle prick, while others may feel moderate pain, especially if the vein is difficult to locate. It is common for the spot to throb after the procedure. Some bleeding is also common, as the needle will break the skin. For most people, bleeding is slight and will not cause any issues.
Infants and Young Children
For infants or young children, the testing process may be different. The doctor may instead make a small cut to allow blood to come to the surface. A test strip or slide will then be used to collect a small sample of blood. The area will be cleaned and bandaged, if necessary.
Your doctor will evaluate the PTH and calcium test results together to assess whether the levels are in balance. If PTH and calcium are balanced, the parathyroid glands are functioning properly. Normal PTH levels are 10 to 55 picograms per milliliter (pg/mL) (Rennert, N. J. 2011).
If PTH levels are low, you may have a condition causing low calcium levels or there could be an issue with your parathyroid glands, causing hypoparathyroidism. If PTH levels are high, you could have hyperparathyroidism, which is commonly caused by a benign parathyroid tumor. If PTH levels are normal and calcium levels are low or high, the issue may not be coming from your parathyroid glands.
PTH levels above 55 pg/mL are considered high (Rennert, N. J. 2011). High PTH levels could indicate:
- conditions that cause increased phosphorus levels, like chronic kidney disease
- the body isn’t responding to PTH (pseudohypoparathyroidism)
- a lack of calcium (this could mean a person is not receiving enough calcium in his or her diet. It could also mean the calcium is not being absorbed or is being lost through urination)
- swelling or tumors in the parathyroid glands
- vitamin D disorders (a person may not be getting enough sunlight or his or her body may be having problems breaking down, absorbing, or using this vitamin. Vitamin D deficiency can lead to muscle and bone weakness)
- pregnancy or breastfeeding (uncommon)
PTH levels below 10 pg/mL are considered low (Rennert, N. J. 2011). Low PTH levels could indicate:
- an autoimmune disorder
- cancers originating from another part of the body that have spread to the bones
- excess calcium has been ingested over a long period of time (from milk or certain antacids)
- low levels of magnesium in the blood
- radiation exposure to the parathyroid glands
- vitamin D intoxication
- sarcoidosis (a disease causing inflammation to tissues)
If either PTH or calcium levels are too high or too low, your doctor may want to do additional testing to more clearly identify the problem.
The risks of PTH measurement include:
- fainting or lightheadedness
- blood accumulating under your skin (hematoma or bruising)