A man experienced severe health complications after losing 30 percent of his body weight in six months using tirzepatide, a new weight loss drug. Researchers at the University of Colorado reported the case in JAMA Internal Medicine. The 62-year-old, who had obesity, Type 1 diabetes, and hypothyroidism, was taking a weight-based dose of levothyroxine. After significant weight loss, he developed atrial fibrillation due to an excess of thyroid hormone.