Resistance to thyroid hormone (RTH) is a syndrome of reduced responsiveness to thyroid hormone caused by mutations in the thyroid hormone receptor beta (TRbeta) gene. Mutant TRbetas exhibit variable degrees of impaired T3 binding resulting in reduced T3-mediated function. The dominant mode of inheritance is attributed to the ability of mutant TRbetas to interfere with the function of the wild-type (WT) TR, a phenomenon known as dominant negative effect (DNE). We recently identified two families with RTH having mutations in amino acid 243 (R243Q and R243W) in whom the mechanism of RTH appears to be distinct from that of other natural TRbeta mutations. These mutations, which are located in the hinge domain of the TRbeta, do not significantly alter the binding affinity for T3, measured in vitro. The present study was undertaken to characterize the properties of these mutant TRbetas to understand the molecular basis of the RTH phenotype. Two other mutant TRbeta producing RTH with mild (320H) and severe (345R) impairment of T3 binding were studied in parallel. The results demonstrate that TRbetas 243Q and 243W could be translocated into the nucleus where they exerted normal ligand-independent repression of positively regulated thyroid hormone response elements. Yet, the addition of 10 nmol/L T3 failed to normalize the transactivation (16-13% of WT) and revert the DNE exerted by the two TRbeta mutants. In contrast, at this T3 concentration, the transactivation function of 320H was significantly higher (50% of WT), and the DNE was completely abolished, in keeping with the mild clinical form of RTH. Formation of 243Q and 243W homodimers on thyroid hormone response elements could not be as readily prevented by T3 as those formed by the WT and 320H TRbetas. These results suggest that the substitution of R243 in TRbeta produces RTH by increasing the propensity for the formation of tightly bound homodimers or by reduction of the receptor affinity for T3 only after it binds to DNA.