Altered thyroxine metabolism in rainbow trout (Oncorhynchus mykiss) exposed to hexabromocyclododecane (HBCD)

Hexabromocyclododecane (HBCD) is an additive flame retardant used in consumer and industrial applications. Our earlier work suggested that fish exposed to HBCD exhibited an increase in the rate of turnover of thyroxine (T4), the precursor molecule to the active thyroid hormone triiodothyronine (T3). To examine this further, juvenile rainbow trout were held in the laboratory and fed diets containing environmentally relevant concentrations of the individual α, β or γ isomers. Thirty-two days after feeding began, 20 fish from each group were gavaged with gelatin containing 1 μCi [125I]-T4. Uptake of [125I]-T4 from the gut was low and recoveries of the initial [125I]-T4 were <10% after 2 d and <3% and <2% after 6 and 14 d respectively, with no differences among groups. However, measurements of the labeled T4 in tissues 2, 4, 6 and 14 d after gavage showed that tissue disposition and elimination rates of the hormone were altered by HBCD. In particular, on day 14 after gavage feeding, there was significantly lower radioactivity in the thyroid of fish exposed to the HBCD isomers relative to the reference group. This implies that either there was lower iodide uptake by the gland in fish exposed to HBCD or that the rate of thyroid hormone turnover in HBCD treated fish was elevated. There was also a significantly higher type II outer ring deiodinase enzyme activity in livers of fish exposed to the β and γ isomers. Taken together, these results indicate that HBCD can potentially impact the thyroid system of fish.