Resistance to thyroid hormone (RTH), a syndrome characterized by variable tissue hyposensitivity to thyroid hormone, is linked to mutations in the thyroid hormone receptor-beta (TR beta) gene. The purpose of this study was to determine whether the clinical phenotypes of RTH can be translated in terms of functional impairment of the corresponding mutant TR beta. Data from 124 subjects with RTH representing 18 different mutant TR beta s, showed that serum free T4 levels correlated with the degree of T3-binding impairment of the corresponding TR beta in 12 of these mutant TR beta s (group I), but not in the remaining 6 (group II). In subjects from both groups studied in detail by the administration of incremental doses of T3, the degree of thyrotroph resistance to T3 correlated with the magnitude of endogenous free T4 elevation at baseline, but did not parallel the resistance of peripheral tissues. In transfection studies, all group I mutant TR beta s inhibited positive transactivation by the wild type TR beta s to a similar degree in the presence of 1 nmol/L T3, whereas group II mutant TR beta s exerted a weaker inhibition that was not related to their T3-dependent trans-activation when tested alone. Similar results were obtained with negatively regulated reporter genes. It is concluded that the clinical severity of RTH, determined by thyrotroph resistance, can be predicted from the degree of T3 binding impairment and dominant negative potency of mutant TR beta s, but the degree of peripheral tissue resistance and related clinical manifestations is limited by putative genetic or environmental factors that modulate the effect of thyroid hormone.