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, phosphorus (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 hypercalcemia or hypocalcemia. This test measures the amount of PTH in the blood.
PTH is produced by four button-sized parathyroid glands that are located in the neck behind the thyroid gland. Normally, these glands secrete PTH into the bloodstream in response to low blood calcium levels. The hormone works in three ways to help raise blood calcium levels back to normal:
- PTH promotes the release of calcium from bones into the bloodstream.
- It stimulates the kidneys to convert vitamin D from the inactive to the active form, which in turn increases the absorption of calcium from food in the intestines.
- It acts on the kidneys to suppress the excretion of calcium in the urine while encouraging excretion of phosphorus.
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)). Intact and fragmented hormone is present in and secreted by the parathyroid gland. The intact hormone represents a smaller fraction, but its portion is increased when calcium levels are low and decreased when calcium levels are high.
Once released into the blood stream, PTH has a very short life span; levels fall by half in less than 5 minutes due to uptake and cleavage in the liver and kidneys. The fragments are referred to as C-terminal fragments and are variably sized, missing anywhere from 6 amino acids to more than half the N-terminal portion of the molecule. C-terminal fragments have a longer half-life, exist in much higher concentrations, and are eventually cleared by the kidneys. Although it was originally thought that the C-terminal fragments were inactive, it now appears that certain fragments may have biologic activities that are able to oppose those of intact PTH.
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.
Is any test preparation needed to ensure the quality of the sample?
Current practices do not require fasting.
PTH levels peak during sleep hours and are lowest during mid-morning to late afternoon hours, the period when most samples are drawn. There may be seasonal fluctuations in PTH due to its inverse relationship with vitamin D. Talk to your health practitioner about sample timing.