Pediatric reference values of TSH should be personalized according to BMI and ethnicity

Objective: The need for personalization of the reference values of thyroid function tests has been previously suggested. We aimed at determining TSH reference values in a large cohort of children according to age, sex, BMI, and ethnicity. Design: A population-based cohort study. Methods: The study cohort included 75 549 healthy children aged 5–18 years. Data analyzed included age, gender, TSH, FT4 levels, BMI and ethnicity. Multivariate logistic regression analysis examined the associations between the study parameters. Results: TSH in the Jewish population is lower than in the non-Jewish population (median: 2.1 IU/L (IQR: 1.5) vs 2.2 IU/L (IQR: 1.5), P < 0.0001). TSH is significantly affected by BMI for children defined as underweight, normal weight, overweight or obese, levels increased as weight diverged from the normal range (median levels: 2.1 IU/L (IQR: 1.4), 2.0 IU/L (IQR: 1.3), 2.1 IU/L (IQR: 1.4), 2.4 (IQR: 1.5), respectively, P < 0.001). The 2.5 percentile is affected by gender and BMI (P < 0.02 and P < 0.001, respectively), while the 97.5 percentile is affected by ethnic origin and BMI (P < 0.001 for both). New TSH reference intervals (RI) adjusted according to BMI and ethnicity are suggested. Comparison of the old and new RI demonstrate the significance of RI personalization: 25.1% of the children with TSH levels above the old RI are within the new RI, while 2.3% of the children who were in the old RI are below the new RI. Conclusions: TSH reference values in children are affected by BMI and ethnicity. Reference values should be individualized accordingly to improve future clinical decision-making and treatment.