How do I know if I am vitamin D deficient?

The most reliable way to know if you are vitamin D deficient is to get a blood test called a 25-hydroxyvitamin D test, ordered by your doctor. A level below 20 ng/mL is generally considered deficient, while 20-30 ng/mL is insufficient. Symptoms that may suggest deficiency include fatigue, bone pain, muscle weakness, frequent illness, depression, and slow wound healing, though many people have no obvious symptoms at all. Risk factors include limited sun exposure, darker skin tone, obesity, older age, and certain medical conditions affecting absorption. If you suspect deficiency, talk to your doctor about getting tested rather than self-diagnosing based on symptoms alone, since those symptoms can have many other causes.