Thyroid hormones affect metabolism via receptors in the cell nucleus however only T3 is active inside the cells as it has a much greater affinity with the nuclear receptors than T4. For this reason T4 can be thought of just as a prehormone used to produce T3.
The thyroid receptors have two functional domains, a central DNA-binding domain and a ligand binding domain.
The thyroid hormones target specific genes, stimulating or repressing them. Examples of genes positively regulated by T3 are fatty acid synthetase and growth hormone.
Negatively regulated genes are stimulated when there is an absence of T3, examples of such genes are TSH and TRH.
Click on graph for negatively regulated genes.