Proper thyroid hormone (TH) action is crucial for almost all organ systems, and disturbances lead to pathophysiological consequences. However, a definition of a 'normal' TSH range and 'normal' T3 and T4 concentrations remains the subject of debates worldwide and has important implications on clinical treatment. Over the last years, new players in TH action were identified such as TH transporters or novel TH metabolites, which contribute to obtaining a more comprehensive picture of TH action. In this issue, new methods of TH metabolite determination as well as their actions on various transmembrane proteins, including trace amine-associated receptor 1 and the TH transporter Lat1, are described. Additionally, TH action in different organ systems such as the brain, gall bladder and adipose tissue as well as gender- and age-dependent aspects are presented. The supplement also discusses epidemiological data on TH metabolites and pathophysiological findings of TH action.weiterlesen