This study focuses on the effects of triclosan (TCS), a ubiquitous antimicrobial agent and aquatic contaminant, on the thyroid of adult zebrafish. The morphology of the thyroid was modified after short term (21 days) dietary exposure of zebrafish to TCS (100 μg g−1 fish per day). Hyperplasia of the thyroid tissue was observed in TCS treated zebrafish, and they had significantly (p < 0.01) more follicles, significantly (p < 0.05) bigger follicles and a significant reduction (p < 0.001) in thyrocyte height relative to the control fish, which is indicative of thyroid inactivation. Analysis of thyroid hormone synthesis associated transcripts in whole zebrafish heads revealed that TCS exposure caused a significant up-regulation of the sodium–iodide symporter (NIS) and thyroid-stimulating hormone (TSH), but did not modify thyroglobulin, thyroid peroxidase or cathepsin Ba. The increase in TSH and NIS transcription coupled to histology indicative of thyroid inactivation suggests that a reduction in circulating thyroid hormones probably occurred, although the exact mechanisms by which TCS reduces thyroid gland activity remains to be established. To our knowledge this is the first study demonstrating that TCS acts on the fish thyroid axis. The importance of the thyroid in basic physiological processes such as metabolism and nervous tissue development means that interference with this axis may have profound consequences for organism health and survival, and the results of the present study highlight the need for more detailed studies of the effects of TCS, which accumulates in sediments and organisms in aquatic environments.
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