Parathyroid Hormone (PTH) & Calcium

Parathyroid hormone, usually shortened to PTH, is the body's main controller of blood calcium. It works on bone, the kidneys, and indirectly the gut to keep calcium within a narrow, tightly defended range.

What PTH is

PTH is a protein (peptide) hormone, not a steroid, so it acts on receptors at the surface of its target cells. Its central purpose is to prevent blood calcium from falling too low. Calcium is essential for nerve signaling, muscle contraction (including the heartbeat), blood clotting, and bone strength, so the body guards its level closely, and PTH is the fastest-acting tool it uses to defend that level minute to minute. When calcium dips even slightly, PTH can be released within moments.

Where it is produced

PTH is made by the parathyroid glands — usually four tiny glands, each about the size of a grain of rice, located on the back of the thyroid gland in the neck. Despite their location and similar name, the parathyroid glands are separate organs from the thyroid and have a completely different job: the thyroid controls metabolism, while the parathyroids control calcium. The glands continuously sample blood calcium and adjust PTH output accordingly.

What it does across body systems

• Bone: It prompts the release of calcium (and phosphate) from bone into the bloodstream when calcium is low. Brief, pulsed exposure and chronic high exposure affect bone differently, which is part of why sustained high PTH can weaken bone over time.
• Kidneys: It signals the kidneys to reclaim calcium from the urine and to excrete more phosphate.
• Vitamin D activation: It encourages the kidneys to convert vitamin D into its active form, which in turn helps the gut absorb more calcium from food — an indirect but important route.
• Net effect: Together these actions raise blood calcium and lower blood phosphate, returning calcium toward its target range.

Regulation and the feedback loop

PTH release is controlled mainly by the calcium level in the blood, sensed directly by the parathyroid glands through calcium-sensing receptors on their surface. When calcium falls, PTH rises; when calcium climbs, PTH is suppressed — a direct, fast negative feedback loop, with calcium itself as the signal rather than a separate pituitary hormone. Active vitamin D and blood phosphate also influence PTH: active vitamin D tends to restrain PTH, while high phosphate tends to drive it up, which is one reason kidney disease (with its phosphate and vitamin D disturbances) so often disturbs PTH. Because PTH and vitamin D interact closely, the two are often considered together. See the related vitamin D page in this section, Vitamin D: The Hormone-Like Vitamin, and the counter-regulating hormone in Calcitonin: Calcium Regulation.

Why the body defends calcium so tightly

It can seem surprising that the body devotes a dedicated hormone and four small glands to a single mineral, but the reason is that blood calcium must stay within an unusually narrow window. Too little calcium makes nerves and muscles overly excitable, which can cause tingling, cramps, and in severe cases dangerous spasms; too much can make them sluggish and can affect the heart, kidneys, and mental state. Bone serves as a vast calcium reservoir, so when intake or absorption falls short, PTH can mobilize calcium from bone to keep the blood level steady. This is useful in the short term but costly to bone if it continues for a long time, which is why chronically high PTH is watched for its effects on bone density. PTH, vitamin D, and the diet together determine whether the body meets its calcium needs from food or has to borrow from the skeleton.

What high or low levels can be associated with

High PTH can be associated with hyperparathyroidism. In primary hyperparathyroidism a gland is overactive, so PTH and calcium are both high. In secondary hyperparathyroidism the glands are responding appropriately to another problem — such as a vitamin D shortage or kidney disease — so PTH is high while calcium is low or normal. Distinguishing these depends on reading PTH against calcium. Low PTH can be associated with hypoparathyroidism, which leads to low calcium and can cause tingling and muscle cramps; it sometimes follows neck surgery that affects the glands. These associations are qualitative; see the conditions index and discuss any concerns with a clinician.

How it relates to other hormones

PTH anchors the calcium-control team. It raises active vitamin D, which then increases calcium absorption from the gut; calcitonin can push calcium in the opposite direction; and phosphate-handling hormones such as FGF23 interact with the same pathways. Because of these links, a clinician evaluating a calcium problem rarely looks at PTH alone, instead reading it alongside calcium, vitamin D, and phosphate. For the wider picture, see the hormones index and the glossary.

How it is measured in blood

PTH is measured from a blood sample, typically with an "intact PTH" assay that detects the active, whole hormone. Because PTH and calcium are read as a pair, the test is usually drawn alongside calcium, and often with vitamin D, phosphate, and kidney-function markers, so the whole calcium system can be interpreted at once. The key to reading PTH is always the matching calcium value. See the blood tests overview and the glossary for related terms.

The patterns above are illustrative only and not diagnostic rules; PTH reference ranges and the calcium pairings that matter depend on the laboratory, the assay, and the individual, and must be interpreted by a clinician. The central idea worth remembering is that PTH is rarely meaningful by itself: it is the combination of PTH with calcium, and often with vitamin D and phosphate, that tells the story.