Vitamin D

Vitamin D is a steroid hormone with an important role in calcium homeostasis and bone formation. The main storage form of vitamin D is 25-hydroxy vitamin D and measurement of this form provides the best assessment of the vitamin D status in the vast majority of patients. There are a small number of clinical settings where measurement of 1,25-dihydroxyvitamin D is the preferred option. In severe vitamin D deficiency rickets may develop. As well as the effects on bone and calcium, vitamin D has been shown to be important for other clinical outcomes such as muscle strength, reduction in falls, reduced risk for diabetes, cardiovascular disease and autoimmune diseases.

Vitamin D can be produced in the skin by exposure to sunlight, obtained as part of a healthy diet or given as oral or parenteral supplements.

Indications for testing

There is uncertainty about the appropriate indications for vitamin D testing at this time. However testing should be considered in patients at risk of reduced sunlight exposure or risk of fracture or with increased demands. Reduced sunlight exposure can occur due to living at extremes of latitude, living primarily indoors (eg some aged-care facility residents), pigmented skin and clothing which covers most of the skin. Elderly patients are at increased risk due to reduced formation of vitamin D in the skin. Patients with osteoporosis, or who are pregnant or breast feeding should also be considered for testing.

Interpretation

There lack of international consensus concerning the ideal levels of vitamin D, however an Australian Position Statement on Vitamin D provides useful guidance.

>50 nmol/L - Vitamin D sufficiency
25 - 50 nmol/L - mild vitamin D deficiency
13 - 25 nmol/L - moderate vitamin D deficiency
<13 nmol/L - severe vitamin D deficiency

SydPath Reference interval: 50 - 150 nmol/L.

There is a marked seasonal variation in vitamin D concentrations with Local data showing that 25 hydroxy vitamin D is generally about 15 nmol/L lower in winer than in summer. Thus a concentration above 65 nmol/L in summer is likely to indicate a concentration above 50 nnmol/L year round.

Based on data from our laboratory, approximately 20% of outpatients have a 25 hydroxy vitamin D concentation in the mild deficiency range; about 4% in the moderate deficiency range and less than 1% in the severe deficiency range.

Note that toxicity is very rare for serum vitamin D below 250 nmol/L and not generally seen below 500 nmol/L.
 

Measurement