Relationship between serum free T4 (FT4) levels and metabolic syndrome (MS) and its components in healthy euthyroid...

The metabolic syndrome (MS) is associated with increased life-time risk for atherosclerotic cardiovascular disease. While some reports suggest that MS and/or insulin resistance is associated with low serum free T4 (FT4) level in euthyroid subjects, this remains a controversial issue.

To clarify the association of serum FT4 level with presence of MS and its components in healthy euthyroid subjects.

For 44,196 euthyroid subjects (including 25,147 males) not taking thyroid hormone or antithyroid-drug and without liver or renal disease who participated in a routine health screening examination, we measured serum FT4, high-density lipoprotein cholesterol (HDL-C), triglycerides (TG), fasting glucose, blood pressure (BP), height, weight, and waist circumference. Subjects were categorized into five groups according to their serum FT4 concentration. The association of FT4 quintiles with clinical and metabolic parameters of MS was analysed by significance test for linearity, and logistic regression analysis was used to calculate odds ratios (ORs) for risk of MS and each of its components, before and after adjustment for age.

In males, serum FT4 concentrations were positively associated with BP, fasting glucose, HDL-C, and TG levels, and negatively associated with waist circumference after adjustment for age (P for trend = 0.033 to < 0.001). Females also showed similar findings except that FT4 had no correlation with TG after adjustment for age (all P for trend < 0.001 except TG). The negative association between FT4 and waist circumference was not present in younger male subjects (< 50 years), and the positive association between FT4 and BP was more prominent in younger female subjects (< 50 years) than in older females (P < 0.05). In both sexes, the ORs for elevated BP and elevated fasting glucose were significantly higher in subjects in the highest FT4 quintile than those in the lowest FT4 quintile, while the ORs for abdominal obesity and low HDL-C were significantly lower. Subjects in the higher FT4 quintiles had a significantly lower prevalence of MS than those in the lowest FT4 quintile, but such differences disappeared after adjustment for age in both males and in females.

These results suggest that the interaction between serum FT4 and MS components could be different according to age as well as gender in euthyroid subjects. Although thyroid hormone significantly affected each component of MS, there was no association between serum FT4 level and presence of MS after adjustment for age in healthy euthyroid subjects.