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BACKGROUND: Resistance to thyroid hormone (RTH) is a syndrome characterized by refractoriness of the pituitary and/or peripheral tissues to the action of thyroid hormone. Mutations in the thyroid hormone receptor beta (TR beta) gene result in TR beta 1 mutants that mediate the clinical phenotype by interfering with transcription of thyroid hormone-regulated genes via a dominant negative effect. In this study, we developed transgenic mice harboring PV, a potent dominant negative human mutant TR beta 1 devoid of thyroid hormone binding and transcriptional activation, as an animal model to understand the molecular basis of this human disease. MATERIALS AND METHODS: Standard molecular biology approaches were used to obtain a cDNA fragment containing mutant PV which was injected into the pronucleus of fertilized egg. Founders were identified by Southern analysis and the expression of PV in tissues was determined by RNA and immunohistochemistry. Thyroid function was determined by radioimmunoassays of the hormones and the behavior of mice was observed using standard methods. RESULTS: The expression of mutant PV was directed by the beta-actin promoter. Mutant PV mRNA was detected in all tissues of transgenic mice, but the levels varied with tissues and with different lines of founders. Thyroid function tests in transgenic mice with high expression of mutant PV showed a significantly (approximately 1.5-fold) higher mean serum total of L-thyroxine levels (p < 0.01) than those of nontransgenic mice. Moreover, thyroid-stimulating hormone levels were not significantly different from those of nontransgenic mice. In addition, these mice displayed decreased weights and a behavioral phenotype characterized by hyperactivity. CONCLUSIONS: These mice have phenotypic features consistent with the commonly observed clinical features of RTH and could be used as a model system to better understand the action of mutant TR beta 1 in a physiological context, which could lead to better treatment for this disease.