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.