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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 (HI5), ten-fold iodine (HI10), fifty-fold iodine (HI50), one hundred-fold iodine (HI100)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 125I-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 T3 and T4 in thyroid tissue significantly decreased, but the T3/T4 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 D 1 activity with increased doses of iodine intakes. There was no significant difference in T4 in thyroid tissue between HI groups and NI group, but a tendency of decrease in T3 level was found in all HI groups.Conclusions In the case of iodine deficiency, D1 activity increased greatly in order to convert more T4 to T3.Excess iodine can inhibit both D1 mRNA expression and its activity to protect organism from being injured by excessive T3.