We report a point mutation in the ligand-binding domain of the TR beta 1 gene in an affected patient and his daughter. The phenotype was borderline hyperthyroid with periodic aggravation of symptoms. In the cognate variant TR beta (TR beta-CN) amino acid codon 322 was exchanged from aspartic acid to asparagine. TR beta-CN revealed strongly decreased T3-binding activity. At low T3 levels TR beta-CN transactivated a palindromic thyroid hormone response element (TRE-PAL) only to a limited extend, whereas full activity was retained at a high T3 concentration. At low T3 levels, TR beta-CN exerted a dominant negative effect on wild-type TR beta, whereas this effect was diminished in the presence of high T3 concentrations. TR beta-CN could not be activated by retinoid X receptor (RXR) beta in the presence of T3, whereas addition of 9cis-retinoic acid (9c-RA) resulted in the transactivation of TRE-PAL through RXR beta independently of the presence of TR beta-CN. In conclusion, the time dependent variable THR phenotype of patient CN might be influenced by the differential expression of RXRs and the T3 and 9c-RA hormonal status.