Serum 25-Hydroxyvitamin D Concentrations

Serum concentration of 25(OH)D is the best indicator of vitamin D status. It reflects vitamin D produced cutaneously and that obtained from food and supplements and has a fairly long circulating half-life of 15 days. However, serum 25(OH)D levels do not indicate the amount of vitamin D stored in other body tissues. Circulating 1,25(OH)₂D is generally not a good indicator of vitamin D status because it has a short half-life of 15 hours and serum concentrations are closely regulated by parathyroid hormone, calcium, and phosphate. Levels of 1,25(OH)₂D do not typically decrease until vitamin D deficiency is severe.

There is considerable discussion of the serum concentrations of 25(OH)D associated with deficiency (e.g., rickets), adequacy for bone health, and optimal overall health (Table 1). A concentration of <15 nanograms per milliliter (ng/mL) (or <37.5 nanomoles per liter [nmol/L]) is generally considered inadequate; concentrations >15 ng/ml (>37.5 nmol/L) are recommended. Higher levels are proposed by some (>30 ng/ml or >75 nmol/L) as desirable for overall health and disease prevention, but insufficient data are available to support them. Serum concentrations of 25(OH)D consistently >200 ng/ml (>500 nmol/L) are potentially toxic.

Table 1: Serum 25-Hydroxyvitamin D [25(OH)D] Concentrations and Health*

ng/mL**	nmol/L**	Health status
<10-11	<25-27.5	Associated with vitamin D deficiency, leading to rickets in infants and children and osteomalacia in adults
<10-15	<25-37.5	Generally considered inadequate for bone and overall health in healthy individuals
≥15	≥37.5	Generally considered adequate for bone and overall health in healthy individuals
Consistently >200	Consistently >500	Considered potentially toxic, leading to hypercalcemia and hyperphosphatemia, although human data are limited. In an animal model, concentrations ≤400 ng/mL (≤1,000 nmol/L) demonstrated no toxicity.

* Serum concentrations of 25(OH)D are reported in both nanograms per milliliter (ng/mL) and nanomoles per liter (nmol/L).
** 1 ng/mL = 2.5 nmol/L

An additional complication in assessing vitamin D status is in the actual measurement of serum concentrations of 25(OH)D. Considerable variability exists among the various assays available and among laboratories that conduct the analyses. This means that compared to the actual concentration of 25(OH)D in a sample of blood serum, a falsely low or falsely high value may be obtained depending on the assay or laboratory used. A standard reference material for 25(OH)D became available in July 2009 that will now permit standardization of values across laboratories.