Parathyroid Hormone (PTH)
PTH (parathyroid hormone) is the hormone that controls the calcium in your blood — when calcium dips it rises to top it up, and eases off when there is plenty. It is almost always read together with a calcium result, because it is the two numbers side by side that tell the story, not either one alone.
Standard CareWhat is parathyroid hormone (pth)?
Parathyroid hormone (PTH) is produced by the parathyroid glands — four small glands that sit behind the thyroid gland at the base of the neck. As healthdirect explains: "your parathyroid glands make parathyroid hormone (PTH). Parathyroid hormone help regulate calcium levels in your blood and bones." PTH's mechanism is straightforward: "PTH works by raising calcium levels in your blood when they drop too low. It does this by adjusting the amount of calcium released by your bones into your blood." At the same time, PTH signals the kidneys to retain more calcium and helps activate vitamin D, which in turn improves how much calcium the gut absorbs from food. The RCPA makes a key point about how PTH is interpreted: "a PTH test is performed together with a calcium test because it is not just the levels of PTH and calcium in the blood that are important, but the balance between them and the response of the parathyroid glands to changing calcium levels." PTH without calcium is like knowing the accelerator position without knowing the speed — you need both to understand what is happening.
Why is it tested?
The RCPA explains that a PTH test "can help diagnose the reason for low or high calcium levels and can show if there is a problem with your parathyroid gland or if it is due to other causes." In practice, this means PTH is almost always ordered because a calcium result has come back outside the reference range and the GP wants to understand why. It may also be ordered when vitamin D is persistently low, or when kidney disease is present (kidney disease affects how vitamin D is activated, which in turn affects calcium and PTH). Often blood tests for phosphate and vitamin D are ordered at the same time.
Reference range (Australia)
PTH is assay-dependent and is not included in the AHRIA harmonised panel. Different laboratories use different assay methods, so the reference interval varies between labs and is always read together with your calcium (and often vitamin D). Use the range printed on your report — that is the interval calibrated to the method your laboratory used.
Reference intervals vary between laboratories. Always use the range printed on your own report.
What your result means
A PTH below the laboratory's reference range, when calcium is also low, can point to underactive parathyroid glands (hypoparathyroidism) — "this leads to low calcium levels in your blood." A low PTH alongside a normal or raised calcium is a different pattern, and is what your GP would expect if calcium is high for reasons outside the parathyroid glands (the glands are correctly suppressing PTH in response). Your GP interprets the direction of both numbers together.
A PTH within your laboratory's reference range alongside a normal calcium is a reassuring combination — it suggests the parathyroid glands are responding appropriately to keep calcium in balance. If your PTH is normal but calcium is outside the range, your GP will consider whether the parathyroid response is appropriate for the situation or whether something else is influencing calcium.
A PTH above the laboratory's reference range usually means the parathyroid glands are working harder than they should need to. The most common explanation is a low or borderline calcium driving the glands to release more PTH to bring it up — this is a normal protective response, often caused by vitamin D deficiency or kidney disease reducing the kidneys' ability to activate vitamin D. As healthdirect notes, overactive parathyroid glands (hyperparathyroidism) "leads to high calcium levels in your blood." Your GP will read the PTH result alongside the calcium and vitamin D to distinguish between these different scenarios.
What can affect your result
PTH responds primarily to blood calcium levels. Anything that lowers calcium — low dietary calcium intake, vitamin D deficiency (which reduces calcium absorption from the gut), or kidney disease (which reduces vitamin D activation) — will push PTH up as the parathyroid glands try to restore calcium. Conversely, anything that raises calcium will suppress PTH. Vitamin D is closely intertwined with PTH. Healthdirect notes: "vitamin D is important for calcium absorption. Vitamin D deficiency can disrupt the calcium balance in your blood, leading to health issues." Low vitamin D is one of the most common reasons for a mildly elevated PTH. Some medicines affect the calcium-PTH axis: medicines containing calcium or vitamin D can suppress PTH; certain diuretics and some other medicines affect how the kidneys handle calcium, and therefore indirectly affect PTH. Magnesium also plays a role — severe magnesium deficiency can impair PTH secretion, which is another reason these markers are sometimes checked together.
When to act
An abnormal PTH result is almost always interpreted in the context of calcium and often vitamin D. If your GP has ordered PTH, they will look at all three numbers together before drawing any conclusions. A PTH that is outside the reference interval on its own — without knowing the calcium — is very hard to act on meaningfully. Most PTH abnormalities that are picked up on routine testing turn out to have straightforward explanations such as vitamin D deficiency, which is common in Australia. If your GP wants to investigate further, they may check phosphate, kidney function, and bone density depending on the overall picture. This page is general information and is not a substitute for advice from your GP.
Frequently asked questions
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