Approaches for assessing the presence and impact of thyroid hormone disrupting chemicals in sea bass (Dicentrarchus labrax) from European coasts

Thyroid functional status of wild fish in relation with the contamination of their environment
deserves further investigation. Polychlorinated biphenyls (PCBs) and organochlorine pesticides
like Dichloro-Diphenyl-Trichloroethane (DDTs), Hexachlorocyclohexanes (HCHs), aldrin,
dieldrin and trace elements (Cd, Cu, Se, Pb, Zn and Hg) were analysed in the muscle of sea
bass (Dicentrarchus labrax) sampled in coastal regions near several important European river
mouths (Gironde, Charente, Loire, Seine and Scheldt). We applied a multi-level approach of
thyroid function assessment. Thyroxine (T4) and triiodothyronine (T3) concentrations in
muscle were analyzed by radioimmunoassay. The activity of hepatic enzymes involved in
extrathyroidal pathways of thyroid hormone metabolism, viz. deiodination, glucuronidation
and sulfation were analyzed. Last, follicle diameter and epithelial cell heights were measured.
We observed changes that lead to an increased conversion of T4 to T3 and lowered thyroid
hormone excretion. The changes in the metabolic pathways of thyroid hormones can be
interpreted as a pathway to maintain thyroid hormone homeostasis. The higher chlorinated
PCBs seemed to be the most implicated in this perturbation. To gain a more integrated
insight, we examined the effect of subchronic exposure to a mixture of Aroclor standards on
thyroid hormone physiology and metabolism in juvenile sea bass. 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. 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 to 1.0 μg Σ7PCBs per g food pellets) increases
thyroid hormone synthesis and stimulates hepatic T4 outer ring deiodinase and T4 sulfation.
Thyroid hormone tissue levels were preserved thanks to the PCB induced changes in T4
dynamics. At 10 times higher concentrations (10 μg Σ7PCBs per g food pellets) an important
depression of T3 and T4 levels could be observed which are apparently caused by degenerative
histological changes in the thyroid tissue. We propose accurate interpretations, by reviewing
the different stages of teleost thyroid function and regulation.