Uncontrolled/untreated maternal hyperthyroidism has been associated with fetal tachycardia. We report a case of right-ventricular (RV) hypertrophy with pericardial effusion related to untreated maternal Graves’ disease. A 33-year-old G4P1021 woman with uncontrolled Graves’ disease presented at 29 weeks gestation with abdominal pain and vaginal bleeding. Fetal echocardiogram showed severe RV hypertrophy and a pericardial effusion. The infant was born prematurely, and initial transthoracic echocardiogram showed severe RV hypertrophy and a small pericardial effusion. The infant had clinical findings consistent with congenital thyrotoxicosis and was treated for this. Follow-up imaging at 4 weeks showed improvement of the cardiac hypertrophy and pericardial effusion. This article describes the presentation of fetal RV hypertrophy with congenital thyrotoxicosis and underscores the importance of screening for this prenatally in mothers with uncontrolled or untreated hyperthyroidism.