Patients with mutations in the thyroid hormone receptor ␤ (TR␤) gene manifest resistance to thyroid hormone (RTH), resulting in a constellation of variable phenotypic abnormalities. To understand the molecular basis underlying the action of mutant TR␤ in vivo, we generated mice with a targeted mutation in the TR␤ gene (TR␤PV; PV, mutant thyroid hormone receptor kindred PV) by using homologous recombination and the Cre͞loxP system. Mice expressing a single PV allele showed the typical abnormalities of thyroid function found in heterozygous humans with RTH. Homozygous PV mice exhibit severe dysfunction of the pituitary-thyroid axis, impaired weight gains, and abnormal bone development. This phenotype is distinct from that seen in mice with a null mutation in the TR␤ gene. Importantly, we identified abnormal expression patterns of several genes in tissues of TR␤PV mice, demonstrating the interference of the mutant TR with the gene regulatory functions of the wild-type TR in vivo. These results show that the actions of mutant and wild-type TR␤ in vivo are distinct. This model allows further study of the molecular action of mutant TR in vivo, which could lead to better treatment for RTH patients. The thyroid hormone 3,3Ј,5-triiodo-L-thyronine (T3) regulates growth, development, and differentiation. Its actions are mainly mediated through thyroid hormone receptors (TRs), which are ligand-dependent transcription factors (1, 2). Three ligand-activated TR isoforms have been identified, TR␣1, TR␤1, and TR␤2, which are derived from the TR␣ and TR␤ genes, respectively. Each TR isoform has a unique developmental and tissue-specific expression (1, 2). TR binds to specific DNA sequences known as thyroid hormone response elements (TRE) in the promoter regions of T3 target genes. The transcriptional activity of TR depends not only on the types of TRE but also on a host of coregulatory proteins (3).RTH is a syndrome characterized by reduced sensitivity of tissues to the action of thyroid hormone. This condition is characterized by elevated levels of circulating thyroid hormones associated with normal or high levels of serum thyroidstimulating hormone (TSH) (4). The most common form of RTH is familial with autosomal dominant inheritance (4). Patients are usually heterozygotes with only one mutant TR␤ gene, and the symptoms are mild. Moreover, clinical manifestations are variable among families with RTH and also among affected family members. Some of the clinical features that have been reported include goiter, short stature, decreased weight, tachycardia, hearing loss, attention deficit-hyperactivity disorder, decreased IQ, and dyslexia (4). One single patient homozygous for a mutant TR␤ has been reported (5). This patient displayed a complex phenotype of extreme RTH with very high levels of thyroid hormone and TSH. Most TR␤ mutants derived from RTH patients have reduced T3-binding affinities and transcriptional capacities. These TR␤ mutants exhibit a dominant-negative effect when cotransfected with wild-type TRs (6, 7). TR␤ mutan...