Hypothyroidism increases the rate of pregnancy loss. Other manifestations include menstrual disorder, and infertility. Serum levels of gonadotropins are low in hypothyroid patients. Though studies of uterine ultrastructure are well established as approaches to investigating the pathophysiology of infertility, they have scarcely been extended to the study of hypothyroid related infertility. The present study investigates the effect of hypothyroidism on the ultrastructure of uterine epithelium. Three groups of Wistar rats were studied. Two groups were initially made hypothyroid using methimazole, and the third group was an untreated control. One hypothyroid group was given daily injections of thyroxine for six weeks. The uteri were removed in all three groups, and processed for transmission electron microscopy and morphometry. It was found that absolute epithelial cell volume was decreased in hypothyroidism. The volume of the nucleus had decreased though its relative volume in the cell had Abstract of the 8th Royan International Twin Congress, Tehran, Iran, 5-7 September 2007 28 Yakhteh Medical Journal, Vol 9, Sup 1, Summer 2007 increased. The height of the luminal epithelium in hypothyroid rats also decreased by (33.8%) as compared with controls. Basement membrane thickness was significantly increased in hypothyroidism. The changes were all substantially abrogated by the administration of thyroxine. This study suggests that thyroid hormones might be importantly concerned in the maintenance of the normal structure of uterine epithelial cells.