Type 1 iodothyronine deiodinase activity and mRNA expression in rat thyroid tissue with different iodine intakes.

Background: Type 1 deiodinase (D1) plays an important role in the metabolism of thyroid hormone and has close relationship with thyroid function. In this study we explore the effects of iodine intake on D1 activity and its mRNA expression and its possible mechanism.

Methods: Forty-eight Wistar rats were randomly divided into six groups with 8 in each: low iodine (LI), normal iodine (NI), five-fold iodine (HI(5)), ten-fold iodine (HI(10)), fifty-fold iodine (HI(50)), one hundred-fold iodine (HI(100)) group. Three months, six months and twelve months after admistration of potassium iodate, they were sacrificed and thyroids were excised. The expression of D1 mRNA in the thyroid tissue was determined by RT-PCR and D1 activity was analyzed by (125)I-rT3 as substrate. The thyroid hormone was measured with radioimmunoassay method.

Results: Compared with NI group, D1 mRNA expression in LI groups slightly decreased, and D1 activity greatly increased. Both T(3) and T(4) in thyroid tissue significantly decreased, but the T(3)/T(4) ratio increased. D1 mRNA expression decreased in all HI groups, and D1 activity was significantly lower in HI groups. There was a tendency of decrease in D1 activity with increased doses of iodine intakes. There was no significant difference in T(4) in thyroid tissue between HI groups and NI group, but a tendency of decrease in T(3) level was found in all HI groups.

Conclusions: In the case of iodine deficiency, D1 activity increased greatly in order to convert more T(4) to T(3). Excess iodine can inhibit both D1 mRNA expression and its activity to protect organism from being injured by excessive T(3).