Parathyroid Hormone (PTH) Blood Test
A parathyroid hormone test, often written as PTH, measures the hormone that regulates calcium in the body. Because PTH and calcium work together, the test is almost always interpreted alongside a calcium level to understand how the system is balancing.
What the test measures
Parathyroid hormone is made by four small parathyroid glands located near the thyroid in the neck. Its main job is to keep blood calcium within a narrow range. When calcium falls, PTH rises to raise it by acting on bones, kidneys, and the gut; when calcium is high, PTH normally falls.
The test usually measures intact PTH, the active form. Because PTH and calcium are part of one feedback system, the relationship between the two values is what matters most, not the PTH number alone.
Why a clinician might order it
A clinician may order PTH when calcium is too high or too low, when evaluating bone disease, kidney conditions, or vitamin D problems, or when investigating symptoms such as fatigue, bone pain, or kidney stones. It is also used to monitor people with known parathyroid or kidney conditions and to understand why calcium is out of range.
How to prepare
Preparation is usually minimal, though some laboratories prefer a morning draw, and PTH is often collected at the same time as calcium. Vitamin D status, kidney function, and several medications can influence PTH, so it helps to share relevant history and a current medication list with the testing team.
How results are generally interpreted
Interpretation depends on the matching calcium level.
- High PTH with high calcium may point toward a parathyroid gland producing too much hormone.
- High PTH with low or normal calcium can reflect the glands responding to low calcium or low vitamin D.
- Low PTH may suggest reduced parathyroid function, especially when calcium is also low.
Illustrative reference ranges
The values below are illustrative only and vary by laboratory and assay. Always use the range printed on your own report and read PTH together with calcium.
| Measure | Illustrative range |
|---|---|
| Intact PTH | ~15-65 pg/mL |
| Calcium (read together) | ~8.5-10.5 mg/dL |
| Interpretation | based on the PTH-calcium relationship |
Related tests
PTH is closely linked with calcium and vitamin D, which are commonly checked together when evaluating bone and mineral health. See the blood tests index or explore related conditions for background.
Frequently asked questions
Why is calcium measured with PTH?
PTH and calcium form a feedback system, so the relationship between the two values is what tells whether the response is appropriate.
Does vitamin D affect PTH?
Yes. Low vitamin D can raise PTH as the glands respond to maintain calcium, which is why vitamin D is often checked too.
Do I need to fast?
Usually not, though some laboratories prefer a morning draw. Follow the instructions you are given.
What can a high PTH mean?
It depends on the calcium level. High PTH with high calcium suggests one cause, while high PTH with low calcium suggests another, so context is essential.
Sources
- MedlinePlus. Endocrine Diseases. https://medlineplus.gov/endocrinediseases.html
- MedlinePlus. Osteoporosis. https://medlineplus.gov/osteoporosis.html
- Endocrine Society. https://www.endocrine.org/