Triclosan and Anuran Metamorphosis: No Effect on Thyroid-Mediated Metamorphosis in Xenopus laevis

Nieuwkoop and Faber stage 51 Xenopus laevis larvae were exposed for 21 days to four different concentrations of triclosan (TCS): < 0.2 (control), 0.6, 1.5, 7.2, or 32.3 mu g TCS/l. Primary endpoints were survival, hind limb length, body length (whole; snout to vent), developmental stage, wet whole body weight, and thyroid histology. Thyroid hormone (TH) concentrations were determined in whole thyroid and plasma samples from stage-matched exposure day 21 specimens. TH receptor-beta (TR beta) expression was measured in stage-matched tail fin tissue samples collected at exposure days 0 and 21. Reduced larval growth occurred at exposure day 21 with 1.5 mu g/l treatment. Larval developmental stage at exposure day 21 was not significantly different from controls based on observed parameters. Thyroid histology was not affected by TCS, and thyroxine (T4) levels in thyroid glands or plasma were not different from controls. A concentration-dependent increase in TR beta expression in exposure day 21 larvae was not detected. However, increased expression was found in stage-matched larvae exposed to 1.5 or 7.2 mu g TCS/l. Our study indicates that environmentally relevant TCS concentrations do not alter the normal course of thyroid-mediated metamorphosis in this standard anuran model.