The current study examines the effect of subchronic exposure to a mixture of Aroclor standards on thyroid hormone physiology and metabolism in juvenile sea bass. The contaminant mixture was formulated to reflect the persistent organic pollution to which the European sea bass population could conceivably be exposed (0.3, 0.6 and 1.0 μg Σ7PCBs per g food pellets) and higher (10 μg Σ7PCBs per g food pellets). After 120 days of exposure, histomorphometry of thyroid tissue, muscular thyroid hormone concentration and activity of enzymes involved in metabolism of thyroid hormones were assessed. Mean concentrations of 8, 86, 142, 214 and 2279 ng g(-1)ww (Σ7 ICES PCB congeners) were determined after 120 days exposure. The results show that the effects of PCB exposures on the thyroid system are dose-dependent. Exposure to environmentally relevant doses of PCB (0.3-1.0 μg Σ7PCBs per g food pellets) induced a larger variability of the follicle diameter and stimulated hepatic T(4) outer ring deiodinase. Muscular thyroid hormone levels were preserved thanks to the PCB induced changes in T(4) dynamics. At 10 times higher concentrations (10 μg Σ7PCBs per g food pellets) an important depression of T(3) and T(4) levels could be observed which are apparently caused by degenerative histological changes in the thyroid tissue.
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