Polybrominated diphenyl ethers (PBDEs) have been used as flame-retardants in consumer products and are currently detected in salmon globally, from farmed Atlantic salmon to wild threatened/endangered Pacific salmon. The two most predominant PBDE congeners found in these salmon are BDE-47 (2,2',4,4'-tetrabromodiphenyl ether) and BDE-99 (2,2',4,4',5-pentabromodiphenyl ether). In the present study, groups of juvenile Pacific Chinook salmon were fed five environmentally relevant concentrations of either BDE-47 (0.3-552 ng total PBDEs/g food), BDE-99 (0.3-580 ng total PBDEs/g food), or nearly equal mixtures of both congeners (0.7-690 ng total PBDEs/g food) for 39-40 days.  The concentrations of circulating total thyroid hormones, thyroxine (T₄) and 3,5,3’-triidothyronine (T₃), were then measured to determine if PBDE exposure disrupts the hypothalamic-pituitary-thyroid endocrine axis. Circulating concentrations of total T₄ and T₃ in the plasma of salmon were determined using a hormone-specific time-resolved fluoroimmunoassay. The concentrations of both T₄ and T₃ were altered by dietary uptake of BDE-99.  Juvenile fish with whole body concentrations of 24.1 ng total PBDEs/g wet weight (ww) had decreased circulating T₄ relative to those with lower whole body concentrations of PBDEs (2.1 and 6.8 ng total PBDEs/g ww). T₃ levels were significantly lower in fish exposed to the greatest concentration of BDE-99 in their diet (219 ng total PBDE/g ww) relative to BDE-99 fed fish with whole body concentrations of 24.1 ng total PBDE/g ww.  Exposure to BDE-47 alone did not alter either T₃ or T₄ circulating hormone concentrations. However, exposure to a mixture of BDE-47 and BDE-99 reduced T₃ in fish with lower concentrations of total whole body PBDEs than with either congener alone at equivalent PBDE whole body concentrations. Accordingly, the disruption of PBDEs on circulating thyroid hormone concentrations has the potential to impact a number of critical functions in juvenile salmon including growth, parr-smolt transformation, and immunological processes.

Highlights

•Polybrominated diphenyl ethers (PBDEs) have been used as flame-retardants in consumer products and are currently detected in salmon globally, from farmed Atlantic salmon to wild threatened/endangered Pacific salmon.

•Juvenile Pacific Chinook salmon were fed five environmentally relevant concentrations of either BDE-47, BDE-99, or nearly equal mixtures of both congeners for up to 40 days.  

•The concentrations of both thyroid hormone thyroxine (T₄) and 3,5,3’-triidothyronine (T₃) were altered by dietary uptake of BDE-99 signaling a disruption to the hypothalamic-pituitary-thyroid endocrine axis.

•Exposure to BDE-47 alone did not alter either T₃ or T₄ circulating hormone concentrations. However, exposure to a mixture of BDE-47 and BDE-99 reduced T₃ in fish with lower concentrations of total whole body PBDEs than with either congener alone at equivalent PBDE whole body concentrations.

•Accordingly, the disruption of PBDEs on circulating thyroid hormone concentrations has the potential to impact a number of critical functions in juvenile salmon including growth, parr-smolt transformation, and immunological processes.