Taking vitamin D blind is common and mostly harmless, but testing first tells you the right dose, and whether you need one.
Aug 5, 2026
5 min read
Dr. Rohan Desai
From October to March, anyone living above about 40° latitude, Boston included, makes almost no vitamin D from sunlight. Summer stores drain through the winter, and by February many people are genuinely low.
The number worth knowing
We aim for 25-hydroxyvitamin D above 30 ng/mL, ideally between 40 and 60. Under 20 is a deficiency worth treating properly. Over 100 is not better; it pushes calcium up and adds nothing useful.
Dose follows the result
Someone at 18 ng/mL needs a different plan from someone at 34, and body weight changes both. That is the case for testing before supplementing: the same capsule does very different things in two people.
Recheck twelve weeks after you start. If the number hasn’t moved, the problem is absorption rather than dose, and that is worth looking into.
Dr. Rohan Desai
Internal medicine, Tinsdale
