Thyroid hormone receptor beta (TRbeta) dysfunction leads to deafness in humans and mice. Deafness in TRbeta(-/-) mutant mice has been attributed to TRbeta-mediated control of voltage- and Ca(2+)-activated K(+) (BK) channel expression in inner hair cells (IHCs). However, normal hearing in young constitutive BKalpha(-/-) mutants contradicts this hypothesis. Here, we show that mice with hair cell-specific deletion of TRbeta after postnatal day 11 (P11) have a delay in BKalpha expression but normal hearing, indicating that the origin of hearing loss in TRbeta(-/-) mutant mice manifested before P11. Analyzing the phenotype of IHCs in constitutive TRbeta(-/-) mice, we found normal Ca(2+) current amplitudes, exocytosis, and shape of compound action potential waveforms. In contrast, reduced distortion product otoacoustic emissions and cochlear microphonics associated with an abnormal structure of the tectorial membrane and enhanced tectorin levels suggest that disturbed mechanical performance is the primary cause of deafness resulting from TRbeta deficiency.