Decreased liver type I 5'-deiodinase and increased brown adipose tissue type II 5'-deiodinase activity in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-treated Long-Evans rats

The effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was studied on the activity of type I 5'-deiodinase (5' DI) in liver and of type II 5'-deiodinase (5' DII) in brown adipose tissue (BAT) microsomes of Long-Evans rats. TCDD decreased the activity of liver 5' DI and increased that of BAT 5' DII. It also reduced the concentration of serum total thyroxine (T-4) without affecting triiodothyronine (T-3). These effects of TCDD on 5'-deiodinases and serum thyroid hormones are compatible with partial hypothyroidism. Based on our results, it is not possible to decide unequivocally whether TCDD affects thyroid hormone status and 5'-deiodination by a direct action or a feedback mechanism. However, these data are important because they demonstrate that TCDD affects thyroid status in a tissue-specific manner and that its effects on thyroid hormone metabolism are much more complex than currently assumed.