Vitamin D (1,25-dihydroxy)

Ranges vary between assays (some labs print 48 to 190 or 60 to 108) and are higher in pregnancy and childhood; use the range printed.

This is the active form of vitamin D, made by the kidneys from the storage form under the control of parathyroid hormone. It acts directly on the gut to absorb calcium. Its level is tightly regulated and does not reflect the body's vitamin D store, so it is not the test used to check for deficiency; the storage form (25-hydroxy) is used for that.

Doctors request it in specific situations: unexplained high calcium, some kidney and inherited conditions of vitamin D handling, and certain inflammatory conditions such as sarcoidosis where the active form is made outside the kidneys. It is a specialist test and is not part of routine screening.

• Overactive parathyroid glands
• Sarcoidosis, tuberculosis and some lymphomas, where immune cells make the active form
• Pregnancy and childhood growth
• Some inherited conditions of vitamin D handling
• Taking the active form or its close relatives as a medicine

• Reduced kidney filtering, because the kidneys make it
• Underactive parathyroid glands
• Very low vitamin D stores
• High phosphate
• Some rare inherited conditions

• It can sit within range when the storage form is low, because the body keeps it steady for as long as it can
• Different assays give quite different numbers
• It is higher in pregnancy and in growing children in the general population

No fasting needed. Read alongside the 25-hydroxy vitamin D, calcium, phosphate and PTH from the same draw.

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