When should Vitamin D 25-OH be retested?

Vitamin D (25-OH) retesting is generally recommended 3 to 6 months after starting supplementation, as levels reach a steady state within that window. For individuals not supplementing or with risk factors (e.g., dark skin, obesity, malabsorption, limited sun exposure), annual testing may be appropriate. Evidence comes from clinical practice guidelines (e.g., Endocrine Society) and observational data; no large RCTs define the optimal interval. Seasonal variation is significant – peak levels occur in late summer, nadir in early spring. Consumer tests (e.g., MyBody-X) provide a snapshot but should be interpreted by a physician. Self-adjusting doses based on a single test can be risky. For deficient (< 20 ng/ml) or toxic (> 100 ng/ml) levels, more frequent monitoring and medical follow-up are indicated. The evidence level is best described as 'practice' (clinical consensus) rather than strong human interventional evidence. Always consider the test's limitations and the need for clinical context.