Hyperthyroidism may affect the mother as well as the developing fetus. The most serious complication from untreated hyperthyroidism is heart disease, specifically heart failure as a result of the heart working overtime in response to the thyroid hormones stimulating the heart to beat faster and faster. A more serious complication called thyroid storm can also happen. A stressful triggering event such as labor, caesarean section or untreated infection can cause the hyperthyroidism to spin out of control. This excess of thyroid hormone can cause death if not diagnosed and treated promptly.