The role of type I and type II 5′ deiodinases on hexachlorobenzene-induced alteration of the hormonal thyroid status

Treatment of male Wistar rats with hexachlorobenzene (HCB) (1000 mg/kg b.w.) for 3-30 days decreases circulating levels of thyroxine (T4) but does not affect triiodothyronine (T3). Time courses were determined for 5′ deiodinase type I (5′ D-I) activity in thyroid, liver, and kidney and 5′ deiodinase type II (5′ D-II) activity in brown adipose tissue (BAT) to test the possibility that increased deiodinase activity might contribute to the maintenance of the serum T3 level. Specific 5′ D-I activity was increased in the thyroid at 21 days and thereafter. No significant changes were observed in the liver, however, total 5′ D-I activity in this tissue was increased at 30 days of treatment as a consequence of liver weight enhancement. HCB decreased kidney 5′ D-I activity after 15 days, and BAT 5′ D-II activity after 21 days of treatment. Total body 5′ D-I activity was significantly increased by 30 days of HCB-treatment. HCB increased the activity of hepatic T4 uridine diphosphoglucuronosyl transferase (UDPGT) in a time-dependent manner, without changes in T3 UDPGT. We propose that increased T4 to T3 conversion in the thyroid and in the greatly enlarged liver may account for the maintenance of serum T3 concentration in hypothyroxinemic HCB-treated rats.