Parathyroid hormone

Ranges differ noticeably between assays (some labs print 1.3 to 9.3 or 1.6 to 7.5), and people with kidney disease are read against different targets; use the range printed.

Parathyroid hormone is made by four small glands in the neck and is the main controller of blood calcium. When calcium falls, PTH rises and acts on bone, the kidneys and, through vitamin D, the gut to bring calcium back up. The test measures the hormone in a blood sample and is only meaningful when read alongside the calcium taken at the same time.

Doctors request it when calcium is outside the range, to work out whether the parathyroid glands are the cause, and routinely in people with long-term kidney disease, where the glands commonly become overactive. It is also used when looking into low vitamin D, kidney stones and thinning of the bones.

• Low vitamin D, the commonest cause of a rise in the general population
• Reduced kidney filtering
• Overactive parathyroid glands
• Low calcium from any cause, as the glands respond
• Lithium tablets
• Low calcium intake or poor absorption from the gut

• High calcium from causes other than the parathyroid glands, such as some cancers or too much vitamin D
• Underactive parathyroid glands, including after neck surgery
• Low magnesium, which stops the glands working properly
• Large amounts of vitamin D or calcium supplements

• Low vitamin D in winter raises it in a large share of the UK general population
• It follows a daily rhythm and rises after a meal, so time of day and fasting state matter
• A sample that is delayed or warm gives a falsely low value with some assays

Usually taken in the morning, fasting, with a calcium and vitamin D from the same draw. Some labs require the sample on ice. A single value means little without the paired calcium.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.