The worst symptoms of thyroid problems can vary depending on the type and severity of the condition. In general, some of the most debilitating symptoms of thyroid problems include extreme fatigue, muscle weakness, difficulty concentrating or thinking, depression, weight changes, and changes in heart rate or rhythm. Additionally, severe cases of hyperthyroidism (too much thyroid hormone) can cause a life-threatening condition called thyroid storm, which can result in fever, rapid heartbeat, and shock. Overall, it's important to seek medical attention if you're experiencing any severe or persistent symptoms of thyroid problems.