Background: Non-alcoholic fatty liver disease (NAFLD) comprises a spectrum ranging from hepatosteatosis through steatohepatitis to fibrosis and irreversible cirrhosis. Previously, we showed that thyroid hormone (TH) supplementation reduced hepatosteatosis in patients with NAFLD and was more effective in older patients.

Objective and hypotheses: We hypothesized that intrahepatic TH concentrations may decrease with age. To investigate this issue, we examined the effects of age on intrahepatic TH concentrations in a mouse model of NAFLD.

Methods: Intrahepatic TH levels and deiodinase activity were measured in 18-24 week old (Young group) and 108-120 week old (Old group) mice. Mice were fed normal chow diet (NCD) or Western diet with fructose in the drinking water for 8 weeks (WDF). Liver TH concentrations (T₄ and bioactive T₃) were measured by LC-MS/MS. Deiodinase 1 (Dio1) and 3 (Dio3) activity was measured by conversion of ¹²⁵I-labelled rT₃ and T₃ respectively. Results were analyzed by two-way ANOVA investigating the effect of age (Young vs. Old), diet (NCD vs. WDF) and the interaction between age and diet.

Results: Intrahepatic T₄ concentrations were significantly lower in old mice fed NCD (p<0.0001) and WDF (p=0.0002) without an interaction effect. Intrahepatic T₃ concentrations also were significantly lower in older mice (p=0.0053) and mice receiving WDF (p=0.0014) without an interaction effect. As previously described Dio1 activity increases with WDF diet (p=0.03) and decreases with age (p=0.0018). Old mice fed WDF had higher Dio1 activity than mice fed NCD; however, their Dio1 activity was lower than young mice fed WDF. Dio3 activity was increased in older mice (p=0.0021) with a significant interaction effect. Interestingly, young mice fed WDF had increased Dio3 activity whereas old mice had decreased Dio3 activity.

Conclusions: Age and WDF are both factors that decrease intrahepatic TH levels. The decrease in intrahepatic T₃ concentration in old mice fed WDF could explain why TH supplementation was more effective in reducing hepatosteatosis in older patients.