Objective: The thyroid gland has important role in metabolic activities in the body. It has been known that thyroid hormones have a basic role in development of the fetus. Thyroid disorders, including hyperthyroidism are the most common endocrine disorder. The aim of this study was to investigate the effects of maternal hyperthyroidism on the development of thyroid tissue in the rat embryo. Materials and Methods: Fourteen Sprague-Dawley female mature rats weighting 200 ± 20g were used in the study. Female rats were mated with normal male rats and randomly divided into 2 groups. One group as control did not receive any injection. Another group as experimental group received Levothyroxine (L-thyroxine) (0.5 mg/kg per day) by intra peritoneal injection during the gestational period. Serum concentrations of triiodothyronine (T3) and tetraiodothyronine (T4) increased in 2nd group hyperthyroid pregnant rats after two weeks of the start of treatment. Animals were killed on 21th day of gestation and the embryos were taken and their neck and upper portion of thorax were cut and fixed in Boin’s solution for 72 hours. The specimens were then dehydrated and embedded in paraffin. The paraffin blocks were transversely sectioned at 5 micrometer thickness and then stained by hematoxylin-eosin method. Quantitative computer-assisted morphometric study of the follicular surface and colloid surface of each follicle were measured. Number of follicular cells was counted in 1000 square micrometer of each follicle. Statistical analysis of the results was down by using Independent sample t test. Results: Many histological findings in both experimental and control groups revealed that more than 90 percent of thyroid gland surface occupied by follicle and another remaining 10 percent is included by connective tissue and blood vessels. Cell membrane of follicular cells was not detectable in two groups. Cytoplasm and the space between nucleuses in control group were higher than the experimental group. Nucleolus and nuclear membrane was determined and the nucleus was euchromatin and had circular to oval shape in groups. Mean number of follicular cells was significantly increased in experimental group than control group (p<0.05). No significant difference was shown in the mean ratio of colloidal surface to follicular surface in experimental group when compared to control group. Conclusion: The maternal hyperthyroidism can lead to increase in number of follicular cells in the thyroid gland of embryo. Therefore, pregnant women with hyperthyroidism should appropriately be managed.