Conflicting results have been reported regarding the effect of triiodothyronine (T3) on serum leptin and adipose tissue leptin gene expression in human and animals. The aim of the present study was to evaluate the effect of administration of increasing doses of T3 on serum leptin concentration and on leptin mRNA abundance in white adipose tissue of rats. The results presented in this paper indicate that administration of single different doses of T3 to euthyroid rats resulted dose dependent increases of serum total T3 concentrations which are associated with a decrease in white adipose tissue leptin mRNA level. The leptin mRNA level in white adipose tissue was negatively correlated with serum total T3 concentration (r=−0.8, p<0.001). Like white adipose tissue leptin mRNA level, serum leptin concentration decreased after T3 administration, and was also negatively correlated with the serum T3 concentration (r=−0.8, p<0.001). In contrast, administration of T3 to the same rats led to a significant increase in white adipose tissue expression of the malic enzyme gene (malic enzyme activity and malic enzyme mRNA level), a known target gene for T3.
The results indicate that T3 exerts a selective inhibitory effect on white adipose tissue leptin gene expression in vivo. A conclusion is that T3 decreases rat serum leptin concentration by down-regulation of leptin gene expression in white adipose tissue.