Carol felt tired. She felt tired all the time. She didn't use to feel tired. She used to have a lot of energy. Why was she so tired all the time? "You should eat more sugar," her friend told her. "Sugar will give you energy." But Carol didn't want to eat more sugar. More sugar would give her more weight. She didn't want to put on weight. She wanted to have energy again. She finally decided to see a doctor. Her doctor told her that she had a thyroid problem. "What is a thyroid?" she asked her doctor. "It can make you feel very tired. But don't worry, I can fix it," he told her.