Lowering of Serum Thyroxine and Triiodothyronine Levels in Yearling Coho Salmon, Oncorhynchus kisutch, by Dietary Mirex and PCBs

Abstract

Yearling coho salmon, Oncorhynchus kisutch, were fed diets contaminated with PCBs (500 μg or 50 μg/g dry feed), Mirex (50 μg or 5 μg/g dry feed), or a mixture of Mirex and PCBs (5 μg Mirex + 50 μg PCB/g dry feed) daily for 2 or 3 mo. Serum T3 and T4 levels were significantly reduced in fish fed the high Mirex diet for 2 or 3 mo. Serum T3 levels were reduced in fish fed the high PCB diet for 3 mo. Serum T3 and T4 levels were significantly reduced in fish fed the mixed Mirex–PCB diet for 3 mo. The significant increase in serum T4/T3 ratios in fish given the high Mirex or high PCB diet was not evident in fish given the mixed Mirex–PCB diet, suggesting a synergistic effect that is different from the effect of individual contaminants. Fish given the high PCB or high Mirex diet had significantly lower body weights than the control fish. Key words: organochlorines, teleost fish, thyroid gland, hormones, endocrines, synergism, pituitary gland