At a Glance
Why Get Tested?
To determine whether PTH levels are responding normally to changes in blood calcium levels; to distinguish the cause of calcium imbalances, and to evaluate parathyroid function. In some centres, PTH levels may be measured during surgery for hyperparathyroidectomy. This will confirm removal of the parathyroid gland(s).
When to Get Tested?
When calcium blood levels are higher or lower than normal, and when your doctor wants to determine how well your parathyroid gland is functioning.
A blood sample taken from a vein in your arm.
The Test Sample
What is being tested?
Parathyroid hormone (PTH) helps the body maintain stable levels of calcium in the blood. It is part of a ‘feedback loop’ that includes calcium, PTH, vitamin D, and to some extent phosphate and magnesium. Conditions and diseases that disrupt this feedback loop can cause inappropriate elevations or decreases in calcium and PTH levels and lead to symptoms of hypercalcaemia (raised blood levels of calcium) or hypocalcaemia (low blood levels of calcium).
PTH is produced by four parathyroid glands that are located in the neck beside the thyroid gland. Normally, these glands secrete PTH into the bloodstream in response to low blood calcium levels. Parathyroid hormone then works in three ways to help raise blood calcium levels back to normal. It takes calcium from the body’s bone, stimulates the activation of vitamin D in the kidney (which in turn increases the absorption of calcium from the intestines), and suppresses the excretion of calcium in the urine (while encouraging excretion of phosphate). As calcium levels begin to increase in the blood, PTH normally decreases.
Parathyroid hormone itself is composed of 84 amino acids (sometimes called PTH (1-84)). Once it is released from the parathyroid gland into the blood stream, it has a very short life-span; levels fall by half in less than 5 minutes. The fall is caused primarily by the breakdown of PTH to smaller fragments.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Ask a Laboratory Scientist
NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.