In the present study,we investigated the effects of hypothyroidism on the morphology of astrocytes and microglia in the hippocampus of Zucker diabetic fatty rats and Zucker lean control rats.To induce hypothyroidism,Zucker lean control and Zucker diabetic fatty rats at 7 weeks of age orally received the vehicle or methimazole,an anti-thyroid drug,treatment for 5 weeks and were sacrificed at 12 weeks of age in all groups for blood chemistry and immunohistochemical staining.In the methimazole-treated Zucker lean control and Zucker diabetic fatty rats,the serum circulating triiodothyronine(T3)and thyroxine(T4)levels were significantly decreased compared to levels observed in the vehicle-treated Zucker lean control or Zucker diabetic fatty rats.This reduction was more prominent in the methimazole-treated Zucker diabetic fatty group.Glial fibrillary acidic protein immunoreactive astrocytes and ionized calcium-binding adapter molecule 1(Iba-1)-immunoreactive microglia in the Zucker lean control and Zucker diabetic fatty group were diffusely detected in the hippocampal CA1 region and dentate gyrus.There were no significant differences in the glial fibrillary acidic protein and Iba-1 immunoreactivity in the CA1 region and dentate gyrus between Zucker lean control and Zucker diabetic fatty groups.However,in the methimazole-treated Zucker lean control and Zucker diabetic fatty groups,the processes of glial fibrillary acidic protein immunoreactive astrocytes and Iba-1 immunoreactive microglia,were significantly decreased in both the CA1region and dentate gyrus compared to that in the vehicle-treated Zucker lean control and Zucker diabetic fatty groups.These results suggest that diabetes has no effect on the morphology of astrocytes and microglia and that hypothyroidism during the onset of diabetes prominently reduces the processes of astrocytes and microglia. In the present study, we investigated the effects of hypothyroidism on the morphology of astrocytes and microglia in the hippocampus of Zucker diabetic fatty rats and Zucker lean control rats. T induce hypothyroidism, Zucker lean control and Zucker diabetic fatty rats at 7 weeks of age orally received the vehicle or methimazole, an anti-thyroid drug, treatment for 5 weeks and were sacrificed at 12 weeks of age in all groups for blood chemistry and immunohistochemical staining. In the methimazole-treated Zucker lean control and Zucker diabetic fatty rats, the serum circulating triiodothyronine (T3) and thyroxine (T4) levels were significantly decreased compared to levels observed in the vehicle-treated Zucker lean control or Zucker diabetic fatty rats. This reduction was more prominent in the methimazole-treated Zucker diabetic fatty group. Glial fibrillary acidic protein immunoreactive astrocytes and ionized calcium-binding adapter molecule 1 (Iba-1) -immunoreactive microglia in the Zucker lean cont rol and Zucker diabetic fatty group were diffusely detected in the hippocampal CA1 region and dentate gyrus. There were no significant differences in the glial fibrillary acidic protein and Iba-1 immunoreactivity in the CA1 region and dentate gyrus between Zucker lean control and Zucker diabetic fatty groups . Even, in the methimazole-treated Zucker lean control and Zucker diabetic fatty groups, the processes of glial fibrillary acidic protein immunoreactive astrocytes and Iba-1 immunoreactive microglia, were significantly decreased in both the CA1region and dentate gyrus compared to that in the vehicle- treated Zucker lean control and Zucker diabetic fatty groups. These results suggest that diabetes has no effect on the morphology of astrocytes and microglia and that hypothyroidism during the onset of diabetes prominently reduces the processes of astrocytes and microglia.