Studies showed that even in euthyroid individuals (defined as a TSH level in the upper half of the ‘normal’ range), advanced liver disease has been associated with increased conversion of fT4 to inactive metabolite-reverse-triiodothyronine (rT3) in the liver, at the expense of DIO1-mediated conversion to the active metabolite-fT3 [19–21]. The gene discussed is DIO1; the disease is liver disorder.