Objective To study a rare case of spontaneous ovarian hyperstimulation syndrome in a naturally conceived pregnancy associated with overt hypothyroidism. Design Case report. Setting Endocrinology private practice. Patient(s) A 32-year-old woman who was 13 weeks pregnant with bilaterally enlarged ovaries presumed to be secondary to hypothyroidism. Intervention(s) Administration of levothyroxine and titration of the dose. Main Outcome Measure(s) Regression of signs and symptoms of spontaneous ovarian hyperstimulation syndrome after 12 weeks of therapy. Result(s) The patient was diagnosed with severe hypothyroidism, as confirmed by her elevated thyroid-stimulating hormone level. Ultrasound evaluation revealed ovarian enlargement secondary to multiple contiguous cysts with anechoic content. The patient was administered levothyroxine 175 μg/day. Results of hormonal studies demonstrated thyroid function normality at week 12 after treatment. Incomplete regression of ovarian cysts was also noticed within this period. At week 37, the patient developed preeclampsia, and cesarean delivery was recommended. An 8-month postpartum ultrasound evaluation revealed complete regression of the cysts. Conclusion(s) Spontaneous ovarian hyperstimulation syndrome secondary to hypothyroidism may be the cause of ovarian enlargement, and levothyroxine replacement seems an appropriate primary therapeutic option. Proper endocrinological assessment of patients is recommended as it may avoid unfavorable outcomes.