What are the symptoms of an overactive thyroid?

DEAR DOCTOR K: My doctor thinks I may have an overactive thyroid. What does that mean? Which of my symptoms did it cause?
DEAR READER: Hyperthyroidism, or overactive thyroid, is a condition in which your body makes too much thyroid hormone. Thyroid hormones are made by the thyroid gland, which sits just under the skin of the lower front part of your neck.