Is free triiodothyronine important in the development of insulin resistance in healthy people?

Objective To determine the association between thyroid hormones and insulin resistance in a population of healthy individuals. Materials and methods We conducted an analytical cross-sectional study in adults who attended a private clinic from 2012 to 2014. We excluded those participants with fasting glucose values ​​compatible with diabetes mellitus, abnormal thyroid hormone values, chronic use of corticosteroids, and incomplete medical records. Participants were divided into tertiles (low, intermediate and high) according to their free triiodothyronine and free thyroxine values. We defined Insulin resistance as a Homeostasis Model Assessment (HOMA-IR) value greater than 3.8. We conducted a univariate and multivariate Poisson regression model to assess the association between thyroid hormones and insulin resistance. The association measure reported was the prevalence ratio (PR) with their confidence interval (CI) at 95%. Results We evaluated 600 participants. The mean age was 36.8 ± 14.2 years and 33% were male. The frequency of insulin resistance was 29.5%. In the univariate regression, we found association between free triiodothyronine tertiles and insulin resistance. In the multivariate regression adjusted for age, sex, body mass index and thyroid stimulating hormone, the association between free triiodothyronine tertiles and insulin resistance remained; intermediate tertile (PR = 1.54; CI95%: 1.10-2.15) and high tertile (PR = 1.70; CI95%: 1.21-2.39). We found no association between T4 and insulin resistance. Conclusions High levels of free triiodothyronine are associated with insulin resistance. The use of free trioodothyronine to assess insulin resistance in healthy patients should be considered.