Vitamin D deficiency
One of the most common nutritional deficiencies worldwide, easily corrected with supplementation once recognised.
What it is
Vitamin D deficiency develops when sun exposure plus dietary intake fail to maintain stores. Roughly 1 billion people worldwide carry deficient or insufficient levels. The clinical consequences include reduced calcium absorption (predisposing to osteoporosis and fracture), muscle weakness, and a long list of correlated — though not all causally established — associations with cardiovascular, immune and mood health. Diagnostic anchor: 25-OH vitamin D blood test. The active 1,25-OH form is rarely useful for screening.
Key lab markers
- 25-OH vitamin D — the standard storage form measured in screening.
- Calcium — to rule out coexisting hypercalcaemia before high-dose replacement.
- PTH — secondary hyperparathyroidism is the classic consequence of severe deficiency.
Symptoms
Often asymptomatic; severe or chronic deficiency may present with:
- Bone pain or tenderness
- Muscle weakness, especially proximal
- Easy fracturing
- Fatigue
- Low mood
- In children: rickets (deformity of growing bones)
When to discuss with a doctor
25-OH vitamin D below 20 ng/mL is the deficiency threshold and warrants supplementation, generally 2000–5000 IU/day for 8–12 weeks then a maintenance dose. Levels 20–30 ng/mL are insufficient — many clinicians supplement here too. Above 100 ng/mL is the toxicity-risk zone, generally seen only with high-dose supplementation without monitoring. Discuss dose and duration with your clinician.