Thyrotropin Levels, Insulin Resistance, and Metabolic Syndrome: A Cross-Sectional Analysis in the Brazilian Longitudinal Study of Adult Health (ELSA-Brasil).

Background: Previous studies have described an association with thyrotropin (TSH) levels, insulin resistance, and metabolic syndrome. We performed a cross-sectional analysis to investigate the relationship between TSH levels, insulin resistance, and metabolic syndrome using baseline data from the Brazilian Longitudinal Study of Adult Health (ELSA-Brasil).

Methods: Diabetics and individuals using medications that interfere in thyroid function were excluded, leaving 10,935 participants (54.3% women) for current analyses. Homeostasis Model Assessment of Insulin Resistance (HOMA-IR) values above the 75th percentile was considered as indicative of presence of insulin resistance. Logistic regression models were built using HOMA-IR and metabolic syndrome as the dependent variable, and quintiles of TSH as the independent variable (first quintile as reference). Odds ratios (OR) were presented with multivariate adjustment for socioeconomic/cardiovascular risk factors for insulin resistance, and adjustment only for socioeconomic factors and smoking for metabolic syndrome.

Results: Age, body mass index, waist measurement, fasting glucose and fasting and post load insulin and HOMA-IR increased according to TSH quintiles. Subjects in the fifth TSH quintile presented an OR of association with insulin resistance of 1.86 [95% confidence interval (95% CI) 1.26-2.75], regardless of gender. For the metabolic syndrome, subjects in the fifth quintile presented an OR of 1.21 (95% CI 1.01-1.45) and remained positive only for men (OR 1.37; 95% CI 1.07-1.76). Restricting the analysis to quintiles of TSH in the normal range did not change the results.

Conclusions: In this cross-sectional evaluation, high TSH quintiles were associated to insulin resistance/metabolic syndrome.