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; to evaluate parathyroid function; during surgery for hyperparathyroidism, to confirm removal of the gland(s) causing the problem
When to Get Tested?
When blood calcium levels are higher or lower than normal; when your doctor wants to determine how well your parathyroid glands are functioning.
A blood sample drawn from a vein in your arm.
Test Preparation Needed?
None. PTH levels vary during the day so samples are usually collected about 8 am.
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, mainly PTH (35-84) and PTH (7-84). Although it was originally thought that these fragments are inactive, they are, in fact, active but have different actions than PTH (1-84). These fragments, (especially PTH (7-84), seem to do the opposite of PTH: they lower blood calcium levels and prevent removal of calcium from the bone.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
Is any test preparation needed to ensure the quality of the sample?
No special preparation is needed for this test. However, since PTH levels vary during the day, they are usually collected about 8 a.m. Talk to your doctor about sample timing.
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.