Generalized resistance to thyroid hormone (GRTH), is a syndrome of reduced tissue responsiveness to thyroid hormone. So far, mutations linked to GRTH have been only detected in the hormone-binding domain of the human thyroid hormone receptor (hTR)-beta gene. Although there is no doubt that these mutations result in abnormal hTRs, there is a conspicuous lack of correlation between the severity of clinical manifestations and the degree of functional impairment of the mutant hTRs. In this work we examined whether variable expression of mutant genes relative to the normal genes could explain the observed discrepancies. The relative amounts of mutant and normal hTR beta and normal hTR alpha messenger RNAs in fibroblasts from normal subjects and those from individuals with GRTH were estimated by coamplification of their complementary DNA products. Heterozygous subjects with GRTH from two families manifesting differences in the severity of clinical manifestations expressed equally both normal and mutant hTR beta alleles. Furthermore, there was no compensatory increase in the expression of the normal hTR alpha gene in these individuals nor in fibroblasts from members of a third family with homozygous deletion of the hTR beta gene. In vitro treatment with thyroid hormone did not affect the results. It is concluded that the apparent discrepancies between the functional impairment of the mutant hTRs and the clinical manifestations of GRTH are not due to quantitative differences in the expression of the normal or mutant hTR genes but more likely to variations in the interactions of the mutant hTRs with the normal hTR beta, hTR alpha and nuclear stabilization factors.