Hypothyroidism in pregnancy is associated with serious maternal and fetal risk. Rarely, it is manifested by life-threatening cardiac complications, such as gross pericardial effusion and tamponade. We present a case of successfully treated gross pericardial effusion and tamponade in a 22-week pregnant woman with hypothyroidism. The patient was treated by pericardial drainage with further treatment of hypothyroidism with levothyroxine. During the follow-up pregnancy was uncomplicated without recurrence of pericardial effusion and successful delivery of full-term baby. We conclude that careful monitoring of thyroid functional tests and proper management should be performed in pregnant women with hypothyroidism to prevent cardiac complications of the disease, like pericardial effusion and tamponade.