Thyroid hormone (TH) homeostasis depends on peripheral activation and inactivation of iodothyronines by selenoenzymes of the deiodinase (Dio) family. We genetically inactivated hepatic selenoenzyme expression, including Diol, in order to determine the contribution of hepatic Dio to circulating TH levels. Serum levels of TSH, total T-4, and total T-3 were not different from controls. We measured Dio1 and Dio2 in kidney, skeletal muscle, heart, brown adipose tissue, and brain, but did not find compensatory up-regulation in these tissues. Finally, we determined expression in the liver of the following T-3 target genes: Spot14, alpha-glycerophosphate dehydrogenase (alpha GPD), and malic enzyme (ME). On the transcript level, both Spot14 and alpha GPD were reduced in Dio-deficient liver to about 60-70% of controls. However, mRNA and activity of ME were significantly increased in the same mice. Together, our results indicate that hepatic Diol activity is not absolutely required to sustain the euthyroid state in mice. (c) 2005 Elsevier Inc. All rights reserved.