The thyroid hormone receptors (TR) have three major isoforms, TRalpha1, TRalpha2, and TRbeta1; these are ligand-dependent nuclear transcription factors. THRB, the gene encoding TRbeta1, is considered a potential cancer suppressor. The mechanism of its inactivation is not yet clear. Aberrant silencing of THRB in breast cancer tissue and plasma by promoter hypermethylation was investigated in the present study. Reverse transcriptase-polymerase chain reaction (RT-PCR) was used to examine THRB mRNA expression in the breast cancer tissues. Methylation-specific polymerase chain reaction (MSP) combined with nested PCR was used to determine the methylation status of the THRB gene promoter region in 40 cancer tissue and 40 plasma samples from breast cancer patients. Methylation status of MSP product in plasma was also evaluated by direct sequencing. The expression of THRB mRNA in breast cancer tissues was lower than that in the normal tissues; hypermethylation was found in 32 of 40 breast cancer tissues (80%) and in 28 of 40 plasma samples (70%). Loss of THRB gene expression was associated with the CpG island hypermethylation of promoter regions. THRB gene CpG island methylation was not related to clinical pathologic parameters. Sequencing results were identical to agarose gel electrophoresis results. The present results indicate that hypermethylation of THRB as an alternative gene silencing mechanism is highly prevalent in breast cancer. Methylated tumor-specific DNA may serve as a plasma biomarker for prognosis in patients with breast cancer.