How do I know if I have a vitamin D deficiency?

The most reliable way to know if you have a vitamin D deficiency is through a blood test called the 25-hydroxyvitamin D test, ordered by your doctor. This measures the level of vitamin D in your blood, with deficiency generally defined as below 20 ng/mL. Symptoms that might suggest deficiency include fatigue, bone pain, muscle weakness, frequent illness, and depression, though many people have no obvious symptoms at all. Risk factors include limited sun exposure, darker skin tone, obesity, older age, and conditions that affect fat absorption like Crohn's disease. If you suspect a deficiency, talk to your doctor about getting tested rather than self-diagnosing based on symptoms alone.