Determinants of Serum Biotin (Vitamin B7) Concentrations: Influence of Age, Sex, and Season in a Large Population Cohort: A Retrospective Cross-Sectional Study

Background/Objectives: Population-level data on serum biotin (vitamin B7), an essential carboxylase cofactor, remain limited, particularly in Middle Eastern populations. We described serum biotin distribution and examined demographic and seasonal correlates in a large Saudi cohort. Methods: This retrospective cross-sectional study included 11,735 adults (79.0% female; mean age 36.5 ± 11.8 years) with serum biotin measurements from an institutionally supported outpatient clinical/laboratory testing program in Riyadh. Serum biotin was measured by competitive enzyme-linked immunosorbent assay (ELISA); results above the measuring range were recorded at its upper limit (1100 ng/L). Associations were evaluated using non-parametric tests and multivariable regression, including regression for censored data. Results: Mean serum biotin was 351.9 ± 251.8 ng/L (median: 270.9 ng/L). By concentration band, 0.5% of the results were <100 ng/L, 42.4% were 100–250 ng/L, 50.5% were >250 to <1100 ng/L, and 6.6% were at the upper limit (≥1100 ng/L); excluding the latter, the mean was 299.2 ± 160.2 ng/L. Age showed a weak positive correlation with biotin (ρ = 0.18, p < 0.001). Female sex was associated with higher odds of a result at the upper limit (odds ratio [OR] = 1.93, 95% confidence interval [CI]: 1.55–2.40, p < 0.001), whereas the median concentration was slightly higher in males (287.9 vs. 266.8 ng/L). In the complete calendar year 2024, the seasonal medians were 243.2, 290.7, 308.5, and 262.9 ng/L in winter, spring, summer, and autumn, respectively; this pattern was not reproduced in the first half of 2025. Conclusions: In this large Saudi outpatient cohort, concentrations below 100 ng/L were uncommon, whereas concentrations of 100–250 ng/L were frequent and 6.6% of results reached the upper limit of the assay. Age, sex, nationality, and season were associated with serum biotin, but findings require cautious interpretation because of the retrospective design, clinically requested testing, and absence of key clinical and behavioral covariates. These data provide descriptive information and support future studies incorporating supplementation history, renal function, clinical indication for testing, and functional biomarkers of biotin status.