Thyroid hormone receptors: The challenge of elucidating isotype-specific functions and cell-specific response

► Genetic data indicate different functions for thyroid hormone receptor isotypes. ► Combination of knock-out suggest functional redundancy between the isotypes. ► Isotype specific functions exist in some cell types. ► Thyroid hormone target genes differ, depending on cell type and species. ► Synthetic ligands and environmental pollutants modify thyroid hormone signaling.