Mice with one thyroid hormone receptor (TR) alpha-1 allele encoding a dominant negative mutant receptor (TR alpha1(PV/+)) have persistently elevated serum T3 levels (1.9-fold above normal). They also have markedly increased hepatic type 1 iodothyronine deiodinase (D1) mRNA and enzyme activity (4- to 5-fold), whereas other hepatic T3-responsive genes, such as Spot14 and mitochondrial alpha-glycerol phosphate dehydrogenase (alpha-GPD), are only 0.7-fold and 1.7-fold that of wild-type littermates (TR alpha1+/+). To determine the cause of the disproportionate elevation of D1, TR alpha1+/+ and TR alpha1(PV/+) mice were rendered hypothyroid and then treated with T3. Hypothyroidism decreased hepatic D1, Spot14, and alpha-GPD mRNA to similar levels in TR alpha1+/+ and TR alpha1(PV/+) mice, whereas T3 administration caused an approximately 175-fold elevation of D1 mRNA but only a 3- to 6-fold increases in Spot14 and alpha-GPD mRNAs. Interestingly, the hypothyroidism-induced increase in cerebrocortical type 2 iodothyronine deiodinase activity was 3 times greater in the TR alpha1(PV/+) mice, and these mice had no T3-dependent induction of type 3 iodothyronine deiodinase. Thus, the marked responsiveness of hepatic D1 to T3 relative to other genes, such as Spot14 and alpha-GPD, explains the relatively large effect of the modest increase in serum T3 in the TR alpha1(PV/+) mice, and TR alpha plays a key role in T3-dependent positive and negative regulation of the deiodinases in the cerebral cortex.