Thyroid hormone affects a range of biological processes such as growth, development and metabolism. The biologically active form of the hormone is triiodothyronine (T3); it acts through nuclear receptors TR (thyroid receptor) alpha and beta. TR is usually found as a heterodimer with RXR. Binding of T3 to TR releases the repressor complex from the TR-RXR heterodimer. T3 bound TR-RXR along with a number of co-activators can then regulate gene expression. TR is also involved in a non-genomic action through activation of PI3K pathway. Perturbation of thyroid hormone and its receptors affects various processes including lipid metabolism, carbohydrate metabolism, steroid metabolism, thermogenesis and CNS function.