Thyroid disorders, both in women who wish to conceive and in gravidas, has become a topic of much interest to numerous researchers. Ovarian hyperstimulation syndrome (OHSS) is a potentially life-threatening condition among women undergoing controlled ovarian hyperstimulation (COH). We present a case of thyroid dysfunction in severe OHSS in a patient diagnosed with subclinical hypothyroidism before COH. The dose of L-thyroxine (L-T4) was increased before the procedure in order to reach TSH levels below 2.5 mU/L, and from day 1 of the stimulation the dose of L-T4 was increased by 33%. The patient remained clinically and biochemically euthyroid until day 8 after the embriotransfer (ET). Then, the woman developed severe OHSS, with fluid in the pleural and peritoneal cavity and laboratory evidence of severe OHSS. Laboratory thyroid function tests revealed overt hypothyroidism. L-T4 dose was not increased due to serious clinical condition of the patient. Iodine supplementation was initiated instead. After the symptoms subsided, a period of clinical and laboratory euthyroid state was observed, followed by gestational hyperthyroidism. The L-T4 dose was reduced and iodine supplementation was temporarily ceased. The thyroid function stabilized, while maintaining the L-T4 and iodine supplementation, at 20 weeks of gestation. The patient gave birth by a caesarean section at 37 weeks of gestation and returned to the pre-pregnancy dose of L-T4. To the best of our knowledge, this has been the first report about a patient with thyroid dysfunction in severe OHSS in the Polish literature. On the basis of the presented case and a review of the literature on thyroid dysfunction in women undergoing COH and OHSS, we conclude that clinical signs and biochemical parameters need to be taken into consideration while making therapeutic decision in women with thyroid dysfunction in the course of OHSS. Also, further studies are necessary to elucidate the matter.