We report a 19-year-old woman who was diagnosed as polycystic ovary. Hypothyroidism with a markedly elevated TSH level and an enlarged pituitary gland on MRI were noted. The 123I uptake was decreased to 6.5%. After treatment with thyroid hormone, regression of the enlarged pituitary and the ovarian cysts was observed. In the present case, hypothyroidism was considered to have caused a reversible enlargement of the pituitary gland and concomitant polycystic ovary. We concluded that the polycystic ovary might have resulted from the effects of an excessive amount of TSH on immature ovaries.