The vitamin D metabolite which should be measured to determine vitamin D status is 25(OH)D, (25 hydroxy vitamin D) otherwise known as calcidiol.1 This is the major circulating form of vitamin D.

Using calcitriol, the active form of vitamin D, as a measurement of vitamin D status will be misleading. As a person becomes vitamin D deficient, there is an increase in the production of parathyroid hormone which will increase the renal production of calcitriol, which can often show levels as being normal, or even elevated2

The National Osteoporosis Society (NOS) state that in agreement with the Institute of Medicine (IOM), the following vitamin D thresholds are adopted by UK practitioners in respect to bone health3

  • Serum 25OHD < 30 nmol/L is deficient 
  • Serum 25OHD of 30--50 nmol/L may be inadequate in some people 
  • Serum 25OHD > 50 nmol/L is sufficient for almost the whole population


  1. Testing vitamin D.  Available at
  2. Lips, P. Vitamin D Physiology: Prog Biophys Mol Biol 2006 Sept; 92(1) 4-8. Epub 2006, Feb 28
  3. National Osteoporosis Society (2016). Vitamin D and Bone Health: A Practical Clinical Guideline for Patient Management. Accessed online: