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 bexachlorobenzene (HCB) (1000 mg/kg b.w.) for 3-30 days decreases circulating levels of thyroxine (T-4) but does not affect triiodothyronine (T-3). Time courses were determined for 5' deiodinase type 1 (5' D-I) activity in thyroid, liver, and kidney and 5' deiodinase type H (5' D-H) activity in brown adipose tissue (BAT) to test the possibility that increased deiodinase activity might contribute to the maintenance of the serum T-3 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-H 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 T-4 uridine diphosphoglucuronosyl transferase (UDPGT) in a time-dependent manner, without changes in T-3 UDPGT. We propose that increased T-4 to T-3 conversion in the thyroid and in the greatly enlarged liver may account for the maintenance of serum T-3 concentration in hypothyroxinemic HCB-treated rats. (C) 2004 Elsevier Ireland Ltd. All rights reserved.