Ovarian hyperstimulation syndrome is a rare condition in pregnant women. Most cases are associated with the use of ovulation induction and stimulation medications. Some studies have reported cases of this condition in non-pregnant women or women undergoing ovulation therapy. In this case report, we report the case of a 27-year-old pregnant Saudi woman presenting with a picture of severe spontaneous ovarian hyperstimulation syndrome secondary to severe undiagnosed hypothyroidism. Treatment with Eltroxin (thyroxine) led to complete improvement and regression of ovarian enlargement after empirical titrating thyroxin replacement therapy, which proved the presence of this causation. The diagnosis was confirmed by laboratory and imaging findings, which helped prompt management and prevented complications of unneeded surgical intervention.