Parathyroid hormone (PTH) helps control the amount of calcium in the blood. It is made by small parathyroid glands in the neck. A PTH blood test is usually interpreted alongside a calcium result, because the two normally change in response to each other.
When blood calcium falls, the parathyroid glands normally release more PTH to help raise it. When calcium rises, PTH should decrease. Reading both values shows whether the hormone response fits the calcium level.
Not always. The glands may make extra PTH in response to another problem, such as vitamin D deficiency or kidney disease. In other cases, a problem in the glands themselves causes them to keep releasing too much PTH even when blood calcium is already high.
A low PTH level may also need assessment, particularly when calcium is low. Your clinician considers calcium, kidney function, vitamin levels and the reason for testing before deciding what the pattern means.

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