Free triiodothyronine/free thyroxine ratio rather than thyrotropin is more associated with metabolic parameters in healthy euthyroid adult subjects

Objective: The interrelation between TSH, thyroid hormones and metabolic parameters is complex and has not been confirmed. This study aimed to determine the association of TSH and thyroid hormones in euthyroid subjects and the relationship between thyroid function and metabolic risk factors. Furthermore, this study examined whether thyroid function has predictive power for metabolic syndrome. Design: This is a cross-sectional study that included subjects in a medical health check-up programme at a single institution. Patients: The study included 132 346 participants (66 991 men and 65 355 women) aged over 18 years who had TSH, free T4 (FT4) and free T3 (FT3) levels within the institutional reference ranges. Measurements: Thyrotropin, FT4, FT3 and metabolic parameters including height, weight, waist circumference, blood pressure, serum levels of total cholesterol, triglyceride, high-density lipoprotein cholesterol, insulin and glucose were measured. Results: There was a positive association between FT3/FT4 ratio and TSH in both men and women after adjusting for age, body mass index, smoking status and menopausal status (in women). The FT3/FT4 ratio and TSH were positively associated with risk of metabolic syndrome parameters including insulin resistance. The FT3/FT4 ratio had a greater predictive power than TSH for metabolic syndrome in both men and women. Conclusions: Thyrotropin levels were positively associated with FT3/FT4 ratio within the euthyroid range. The higher FT3/FT4 ratio is associated with increased risk of metabolic syndrome parameters and insulin resistance. FT3/FT4 ratio has a better predictive power for metabolic syndrome than TSH.