Thyroid hormones influence numerous physiological and biochemical functions. The expression of the hormonal effects involves several events. The interaction of T<sub>3</sub> with nuclear receptors, and the stimulation of mRNA production appears to be a major step. Extranuclear binding of thyroid hormones could account for early responses. Plasma membrane receptors may play a role in the cellular uptake of T<sub>3</sub> and the stimulation of amino acids and sugar transport. A direct control of oxidative phosphorylation through binding of T<sub>3</sub> to mitochondrial binding sites has been proposed. The role of cytosolic binding proteins remains unclear. The understanding of the mode of action of thyroid hormones requires a better knowledge of the molecular events occurring at the nuclear level, and the relation between the nuclear and extranuclear binding sites in the hormonal expression.