Thyroid hormone receptors (TRs) are members of the steroid hormone/retinoic acid receptor superfamily, which regulate homeostasis, development, and differentiation. Their transcriptional activity is modulated by the thyroid hormone 3,3',5-triiodo-L-thyronine (T3). The present study evaluated the effect of the availability of cytoplasmic T3 on the modulation of transcriptional responses of the TRs. In human choriocarcinoma JEG-3 and monkey COS-1 cells, the cytosolic thyroid hormone binding protein is a monomer of the tetrameric pyruvate kinase, subtype M2, which does not bind T3. The in vivo monomer-tetramer interconversion is regulated by glucose via fructose 1,6-bisphosphate. At the physiological T3 concentration, lowering the glucose concentration led to an increase in the cellular concentration of the cytosolic thyroid hormone binding protein. By using a transient transfection system, a concomitant reduction in the transcriptional activity of the human beta 1 thyroid hormone receptor was detected in both cell lines. In the absence of glucose, the transcriptional activity of the human beta 1 thyroid hormone receptor in JEG-3 and COS-1 cells was reduced by 65-75% and 90-95%, respectively. However, glucose had no effect on the basal transcriptional activity. These findings demonstrate an important prenuclear step in the modulation of the gene regulating activity of the TRs.