What symptoms or risks would suggest I might be truly deficient?

The honest answer: true deficiency shows up as symptoms, not as a lab number, and the symptoms are bone pain, muscle weakness, and a history of falls or fractures.

The strongest symptom signal is bone pain, especially in the shins. One case-control study of 118 patients with tibial bone pain and tenderness found 75.4% were vitamin D deficient, versus 23.6% of 114 controls, with a dose-response pattern suggesting subclinical osteomalacia [1]. That is a striking gap, but it is one study of a specific symptom, not proof that every ache means deficiency.

Muscle weakness and proximal myopathy are the classic signs. Difficulty climbing stairs, getting up from a low chair, or a waddling gait points to the large muscles of the thighs and hips. This is the textbook presentation of osteomalacia in adults. The evidence for this is older and mostly observational, but it is consistent across decades of clinical teaching.

The risk factors matter as much as symptoms. Dark skin, living above 37 degrees latitude in winter, wearing full coverage clothing, obesity, age over 65, and malabsorption conditions like celiac or Crohn's all raise the odds of true deficiency. If you have none of these and no symptoms, the probability you are deficient is low, and the test is not worth the cost.

The one hard rule stands: if you have bone pain, muscle weakness, or falls, get the 25-hydroxyvitamin D test before supplementing. If you are symptom-free, 400 to 800 IU daily is safe and does nothing harmful, but it is not treating anything.

Confidence: high that bone pain with tenderness is a genuine deficiency signal [1]; moderate that muscle weakness alone predicts deficiency, because the evidence is mostly clinical observation rather than controlled trials.