Thyrotoxicosis may be associated with a variety of abnormalities of liver function. The pathogenesis of hepatic dysfunction in thyrotoxicosis is unknown, but has been attributed to mitochondrial dysfunction. We studied the effect of altered thyroid function on the apoptotic index in rat liver. Extensive DNA fragmentation and significantly increased caspase-3 activity (P < .001) and caspase-9 activation (P < .005) were observed in hyperthyroid rat liver; cell death by apoptosis was confirmed. In hyperthyroid rat liver, 60% of mitochondria exhibited disruption of their outer membranes and a decrease in the number of cristae. These findings, along with significant translocation of cytochrome c and second mitochondriaderived activator of caspases to cytosol (P < .005), suggest activation of a mitochondrialmediated pathway. However, no change in the expression levels of Bcl-2, Bax, and Bcl-x L were found in hyperthyroidism. For in vitro experiments, rat liver mitochondria were isolated and purified in sucrose density gradients and were treated with triiodothyronine (T3; 2-8 M). T3 treatment resulted in an abrupt increase in mitochondrial permeability transition. Using a cell-free apoptosis system, the apoptogenic nature of proteins released from mitochondria was confirmed by observing changes in nuclear morphologic features and DNA fragmentation. Proteins released by 6 M T3 contained significantly increased amounts of cytochrome c (P < .01) and induced apoptotic changes in 67% of nuclei. In conclusion, using in vivo and in vitro approaches, we provide evidence that excess T3 causes liver dysfunction by inducing apoptosis, as a result of activation of a mitochondria-dependent pathway. Thus, the results of this study provide an explanation for liver dysfunction associated with hyperthyroidism. ( T hyroid hormone (TH) affects all tissues and modulates the rate of metabolic activity. Liver damage in hyperthyroid patients has been extensively reported since Habershon's original report in 1874. [1][2][3][4][5] Liver function becomes compromised in 45% to 90% of thyrotoxic patients; in most cases, the changes in the liver are characterized by some degree of fatty infiltration, and by cytoplasmic vacuolization, nuclear irregularity, and hyperchromatism in hepatocytes. 6,7 The pathogenesis of hepatic dysfunction in severe hyperthyroidism is unknown. Possible thyroid-liver interactions include liver damage secondary to the systemic effects of excess thyroid hormone, direct toxic effects of thyroid hormone on the liver, an association of intrinsic liver disease and intrinsic thyroid disease involving autoimmune mechanisms, changes in thyroid hormone metabolism secondary to intrinsic liver disease, and subclinical physiologic effects of thyroid hormone on functions of the liver. 5 The hepatic injury associated with hyperthyroidism varies from mild liver dysfunction associated with nonspecific histologic changes to severe central hepatic ischemia.Ultrastructural and functional changes in mitochondria, such as enlargement, a mass ...