Parathyroid hormone (PTH) is a hormone of the parathyroid glands (small glands behind the thyroid) that controls the level of calcium in the blood.
It works like a thermostat: if calcium falls, PTH rises and brings it back up — by drawing calcium from the bones, boosting absorption in the gut (via vitamin D) and retention in the kidneys. So PTH is always interpreted together with calcium.
Reference ranges
The reference range depends on the lab; always assessed together with calcium:
| Normal | ≈ 15 – 65 pg/mL |
| Primary hyperparathyroidism | PTH ↑ and calcium ↑ |
| Secondary (vitamin D deficiency, CKD) | PTH ↑ with normal/low calcium |
| Hypoparathyroidism | PTH ↓ and calcium ↓ |
What an elevated level means
A high PTH (hyperparathyroidism) comes in two types.
Primary: the glands themselves produce excess PTH (usually from an adenoma) — calcium is high in this case. Secondary: PTH rises in response to a lack of calcium — most often because of vitamin D deficiency or chronic kidney disease. A long-standing high PTH leaches calcium from the bones.
What a low level means
A low PTH (hypoparathyroidism) means the glands produce little hormone, and calcium falls.
Causes: damage to or removal of the parathyroid glands (a common complication of surgery on the thyroid gland), autoimmune processes, severe magnesium deficiency. It shows itself as symptoms of low calcium — tingling and muscle cramps.
How the test is done
PTH is measured in blood from a vein, usually in the morning, and always together with calcium (ideally ionized or albumin-corrected), and with vitamin D.
It is precisely the "PTH + calcium" combination that distinguishes primary hyperparathyroidism from secondary and from hypoparathyroidism.
What affects the value
- The calcium level — the main regulator of PTH (feedback).
- Vitamin D deficiency — raises PTH (secondarily).
- Chronic kidney disease — raises PTH.
- Thyroid surgery — may lower it (gland damage).
- Magnesium deficiency — disrupts PTH secretion.
- Some drugs (lithium, thiazides) — affect the level.
Check calcium and vitamin D
PTH controls calcium; assess your risk of a hidden calcium deficiency:
Frequently asked questions
Why is PTH always tested together with calcium?
Because the meaning of PTH is determined by the calcium level. A high PTH with high calcium is primary hyperparathyroidism, while a high PTH with normal or low calcium is secondary (a response to vitamin D deficiency or kidney disease). Without calcium the result is uninformative.
What does a high PTH with normal calcium mean?
Most often it is secondary hyperparathyroidism — the glands work hard to keep calcium normal. The commonest causes are vitamin D deficiency and chronic kidney disease. Correcting them usually lowers PTH.
Why is a long-standing high PTH dangerous?
It constantly "leaches" calcium from the bones, lowering their density and raising the risk of fractures, and it promotes the formation of kidney stones. So a persistently high PTH calls for finding the cause and treatment.
Related terms
Sources
- Parathyroid hormone (PTH) — MedlinePlus (NIH)
- Physiology, Parathyroid Hormone — StatPearls (NCBI)
- PTH reference ranges by vitamin D status. NCBI/PMC