Thyroid Hormone Levels, even within the Euthyroid Range, are Associated with Lipid Levels

Thyroid dysfunction of any magnitude is associated with dyslipidemia. But the relationship of thyroid hormones with lipid parameters among euthyroid population is still unclear.

This is a cross-sectional study to assess relationship between measured [free T3 (fT3), free T4 (fT4), and TSH] and derived (free T3/free T4 ratio, and TSH index) parameters of thyroid profile with different components of lipid profile among euthyroid population.

We included 100 patients (60 men and 40 women) in this study. The mean free T3, free T4, and TSH levels of our study population were 2.4 ± 0.5 pg/mL, 1.2 ± 0.2 ng/dL, and 3.0 ± 1.6 µIU/mL, respectively. Overall, fT3 had a significant positive correlation with HDL cholesterol (r = 0.4, p = 0.01) and a negative correlation with total cholesterol levels (r = -0.3, p = 0.04). While fT4 (r = 0.3, p = 0.04) and fT3/fT4 ratio (r = 0.5, p = 0.001) showed positive correlation only with HDL levels. In subgroup analysis, positive association of fT3 (r = 0.6, p = 0.008), fT4 (r = 0.4, p = 0.04), and fT3/fT4 ratio (r = 0.8, p = 0.001) with HDL cholesterol was significant only in men. And only in the subgroup with TSH ≥ 3 µIU/mL (N = 48), we found a significant negative correlation of fT3 with total cholesterol (r = -0.5, p = 0.01) and LDL levels (r = -0.7, p = 0.001) and a positive correlation with HDL levels (r = 0.5, p = 0.02).

Among euthyroid subjects, fT3 seems to have a significant and consistent favorable association with lipid levels, especially with HDL cholesterol. This positive association of fT3 with HDL is more marked in men and in subjects with TSH ≥ 3 µIU/mL.