Metabolic and Functional Cross Talk Between the Thyroid and Liver.

The liver and thyroid have complicated effects on each other's functions and metabolic homeostasis in the body. Both hypo- and hyperthyroidism influence hepatic carbohydrate and fat metabolism to regulate circulating glucose, cholesterol, and triglyceride levels. Hypothyroidism and "intrahepatic" hypothyroidism also contribute to the development of hypercholesterolemia and metabolic dysfunction-associated steatotic liver disease (MASLD). Likewise, hepatic dysfunction can modulate thyroid function by reducing thyroid hormone (TH) concentrations and their effects on peripheral tissues. In this review, we examine the impact of thyroid disorders and their treatment on hepatic physiology, metabolism, and pathology, as well as the influence of liver disease on thyroid function. We also describe the clinical and experimental evidence for THs playing significant roles in metabolic conditions such as metabolic syndrome, hyperlipidemia, and MASLD. Additionally, we summarize the current literature on the use of thyromimetics for the treatment of metabolic diseases. Recognizing the effects of the thyroid and THs on hepatic metabolism and fuel utilization, and the liver's role in modulating systemic TH action, is important for optimal clinical management of patients with thyroid and/or liver diseases. New emerging concepts on TH actions in the liver and the efficacy of thyromimetics for the treatment of MASLD have reshaped our understanding of the thyroid-liver relationship and the roles of THs in the pathogenesis and treatment of metabolic diseases.