Low-Normal Free Thyroxine Levels in Euthyroid Male Are Associated with Prediabetes

Abnormal thyroid function is associated with impaired glucose homeostasis. This study aimed to determine whether free thyroxine (FT4) influences the prevalence of prediabetes in euthyroid subjects using a cross-sectional survey derived from the Korea National Health and Nutrition Examination Survey, conducted between 2013 and 2015. We studied 2,399 male participants of >20 years of age who were euthyroid and non-diabetic. Prediabetic participants had lower FT4 concentrations than those without prediabetes, but their thyrotropin concentrations were similar. We stratified the population into tertiles according to FT4 concentration. After adjusting for multiple confounding factors, glycosylated hemoglobin (HbA1c) levels significantly decreased with increasing FT4 tertile, whereas fasting plasma glucose (FPG) levels were not associated with FT4 tertiles (HbA1c, P<0.01 in T3 vs. T1; FPG, P=0.489 in T3 vs. T1). The prevalence of prediabetes was significantly higher in T1 (odds ratio, 1.426; 95% confidence interval, 1.126 to 1.806; P<0.01) than in T3. In conclusion, subjects with low-normal serum FT4 had high HbA1c and were more likely to have prediabetes. These results suggest that low FT4 concentration is a risk factor for prediabetes in male, even when thyroid function is within the normal range.