Free thyroxine concentrations is associated with insulin resistance in euthyroid subjects independently from age, gender, nutritional status, and abdominal visceral fat

Thyroid hormones act synergistically with insulin facilitating glucose disposal and utilization in peripheral tissues eliciting complex interplay between thyroid function and insulin resistance (IR). Both hyper- and hypothyroidism have been reported to be associated with IR and there is growing interest in the potential impact of differences in thyroid function within the normal level on IR. However, little is known about the infl uence of variation in thyroid hormone levels on IR and abdominal visceral fat (VAT), a key determinant of IR, in euthyroid persons. To examine these associations we conduced a cross-sectional study in 1859 euthyroid Caucasian subjects (aged 23-70 years, 72.0% females) with different degrees of body mass index (BMI) and no history of diabetes. Fasting TSH, free thyroxine (FT4) free triiodothyronine (FT3), insulin, glucose, and the level of IR, estimated by the homeostasis model assessment for insulin resistance (HOMA-IR), were measured. VAT, defi ned as the distance between the internal face of the rectus abdominal muscle and the anterior wall of the aorta, was assessed by ultrasonography. Euthyroidism was defined for TSH values between 0.2 and 4.2 mUI/l. According to BMI classifi cation, 20.7% ,39.4% and 39.8% were normal weight, overweight and obese respectively. HOMA-IR and VAT were higher in males than in females (3,3\ub12,3 vs. 2,6\ub12,0 and 7,4\ub12,7 vs. 4,7\ub12,2 mm respectively, P<0.001). After adjusting for age, gender, BMI and VAT, FT4 showed an inverse correlation with insulin and HOMA-IR (p<0.05). Insulin and HOMA-IR were signifi cantly lower across FT4 tertiles. On the contrary, VAT was not signifi cantly different between subjects belonging to the I or II or III tertile of TSH, FT4 or FT3. In conclusion low thyroid function, even in the euthyroid state, seems to predispose to IR and can affect insulin sensitivity independently from age, gender, nutritional status and abdominal visceral fat