Excessive T4 or hyperthyroidism can cause a range of side effects. Common symptoms include weight loss, rapid or irregular heartbeat, tremors, irritability, anxiety, heat intolerance, increased sweating, fatigue, muscle weakness, and changes in menstrual patterns. In severe cases or when left untreated, high T4 levels can lead to complications such as osteoporosis, heart problems, and eye-related issues like bulging or dryness. It is important to seek medical attention if experiencing symptoms of high T4 levels to receive proper evaluation, diagnosis, and treatment.