Effects of tetrabromobisphenol A (TBBA) on thyroid hormone (TH)-dependent morphological development and on TH-regulated biomarkers were investigated using the amphibian Xenopus laevis. Tadpoles were exposed in the Xenopus metamorphosis assay to different TBBA concentrations ranging from 2.5 to 500 microg/L for 21 days. Only the highest TBBA concentration inhibited larval development while the biomarker for negative feedback mechanism, TH receptor beta-mRNA, was not affected. Short-term exposures to TBBA, triiodothyronine (T3), and a combination of TBBA and T3 were performed to examine expression of TH-upregulated biomarkers. TBBA alone increased slightly the expression of TH receptor beta- and basic region leucin zipper transcription factor b/Zip-mRNA but inhibited their T3-induced elevation in a dose-dependent manner. Despite the fact that TBBA seems to cause only mild effects during long-term treatment on larval development, short-term exposure revealed indirect evidence that TBBA can function as a TH antagonist.