Purpose of investigation: To report a rare case of maternal hyperthyroidism after intrauterine insemination due to hypertrophic action of hCG. Materials and Methods: A 36-year-old woman after successful intrauterine insemination and triplet pregnancy, developed hyperthyroidism with resistance to medical treatment. Results: All signs of hyperthyroidism resolved and the results of thyroid function tests returned to normal without any medication after embryo meiosis. Conclusions: De novo maternal hyperthyroidism may develop during pregnancy as a result of pathological stimulation of the thyroid gland from the high levels of hCG hormone that can be seen in multiple pregnancies. The risk of hyperthyroidism is related to the number of fetuses. Reversibility of symptomatology can be seen after fetal reduction of multiple pregnancies.