In this presentation, I present evidence indicating a direct action of thyroid 
hormone at the level of the plasma membrane. Characteristically, the plasma 
membrane-mediated effects of thyroid hormones are prompt in onset, independent 
of new protein synthesis, and are associated with changes in the transmembrane 
transport of ions and substrates. The presence of specific binding sites for 
thyroid hormone in plasma membrane of various tissues and species, although 
inconclusive in itself, provides additional support for the direct action of 
thyroid hormone on the plasma membrane. A model for the mechanism of action of 
thyroid hormone at the plasma membrane level to increase sugar uptake by rat 
thymocytes is delineated, and the physiological role of the plasma 
membrane-mediated action of thyroid hormone is discussed.