Human: I've been told I have a thyroid disorder. What does that mean? Assistant: 
Thyroid disorders are caused by an overactive or underactive thyroid gland. An overactive thyroid gland is known as hyperthyroidism, while an underactive thyroid gland is called hypothyroidism. Symptoms of these conditions can include weight gain or loss, fatigue, high cholesterol levels, and increased heart rate. Treatment for these conditions may involve medication, lifestyle changes such as diet and exercise, or surgery.