Thyroid Axis Disruption in Juvenile Brown Trout (Salmo trutta) Exposed to the Flame Retardant β-Tetrabromoethylcyclohexane (β-TBECH) via the Diet

Abstract

Tetrabromoethylcyclohexane (TBECH) is an additive brominated flame retardant used in domestic and industrial applications. It has been detected in wildlife, and there is early evidence that it is an endocrine disruptor. Whereas other brominated flame retardants with similar physicochemical properties have been shown to disrupt the thyroid axis, no such evaluation has been conducted for TBECH. To elucidate this, juvenile brown trout (Salmo trutta) were fed either a control diet or diets containing low, medium, or high doses of β-TBECH, the isomer most frequently detected in wildlife, for 56 days (uptake phase) followed by a control diet for an additional 77 days (depuration phase). Eight fish per treatment were lethally sampled on uptake days 7, 14, 21, 35, 49, and 56 and on depuration days 7, 21, 35, 49, and 77 to assess fish condition, circulating free and total triiodothyronine and thyroxine, and thyroid epithelial cell height. Although there was no effect on condition factor, there was a significant reduction in total plasma thyroxine in the high dose group and a significant increase in mean thyroid epithelial cell height in the low, medium, and high dose groups during the uptake phase, whereas there were no differences in the depuration phase. These results indicate that β-TBECH may modulate the thyroid axis in fish at environmentally relevant concentrations.