Establishment of Reference Intervals for Plasma TSH Across the Lifespan in an Algerian Mediterranean Population

BACKGROUND: Thyroid-stimulating hormone (TSH) serves as a sensitive marker of thyroid function; however, its interpretation is subject to variation across different populations due to physiological, environmental, and methodological factors. Currently, no validated TSH reference intervals (RIs) have been established for the Algerian population. OBJECTIVE: This study aimed to establish age-specific, assay-validated reference intervals for plasma TSH levels in an Algerian Mediterranean population using a standardized immunometric platform. METHODS: A cross-sectional study was conducted on 3012 apparently healthy participants (1 day to 93 years). After excluding outliers and antibody-positive individuals, 2882 participants were analyzed. Plasma TSH levels were measured using the Mindray CL-900i chemiluminescent assay. Nonparametric percentile methods were applied to estimate age- and sex-specific RIs with bootstrap-derived confidence intervals. RESULTS: Age-specific TSH reference intervals varied markedly across the lifespan. The widest limits were observed in neonates (2.5th-97.5th percentiles: 0.50-22.41 mIU/L), followed by progressive narrowing through childhood and adulthood (19-50 years: 0.54-4.68 mIU/L). In older individuals, the upper reference limit increased modestly, reaching 5.17 mIU/L after age 70. All reference intervals were derived using non-parametric percentile methods following outlier exclusion. CONCLUSION: This study provides the first locally validated age-specific TSH reference intervals for Algeria. Age was the principal determinant of TSH variation, whereas sex-related differences were not clinically significant. The results enhance the diagnostic precision for thyroid disorders and support assay-specific standardization in North African laboratories.