Vitamin D Test

25-hydroxyvitamin D from a simple blood spot collection

In Canada, a lack of direct sunlight in winter means we do not make vitamin D from October to April.

Only about 1/3 of Canadians have vitamin D levels* above 75 nmol/L, the minimum considered optimal for health.  Health Canada

A 2010 study estimated that 37,000 deaths could be prevented every year if the Canadian average vitamin D level* was 105 nmol/L. Grant WB

*measured as 25-hydroxyvitamin D3, the standard test for vitamin D sufficiency. 

Causes of Vitamin D Deficiency

Season or Latitude

The low angle of the sun in Canada means we cannot make vitamin D during the winter months.

Sunscreen Use

SPF 15 sunscreen reduces D synthesis by 99%.

Skin Colour

Dark skin has more of the pigment melanin, which can reduce vitamin D synthesis by up to 99%.


Vitamin D synthesis decreases by about 75% in a 70 year old.


Specific medications, like anti-rejection, anti-seizure and AIDS medicines can speed up the breakdown of vitamin D, resulting in low levels of 25-hydroxyvitamin D. 

Cholesterol-lowering medications are commonly prescribed for high cholesterol and heart disease.   Monitoring cholesterol is important, because having too-low cholesterol impairs the body's ability to make vitamin D.

When to test?

Ideally you want to know what your vitamin D level is after sun exposure, and after little or no sun exposure. 

Many experts recommend testing twice a year - at the end of winter (March) for the lowest levels, and again at the end of August to see levels at their highest.   If you supplement with vitamin D, any time of year is a good time to test, but if you have recently changed your dose of vitamin D, wait at least 4 weeks before testing.