Hyperthyroidism occurs when the thyroid gland makes too much thyroid hormone. Excess thyroid hormone can cause symptoms such as tiredness, weight loss, increased heart rate, heat intolerance, sweating, irritability, anxiety, muscle weakness, and thyroid enlargement.

An autoimmune disorder called Graves' disease is the most common cause of hyperthyroidism. Other causes include growths in the thyroid gland (thyroid nodules), inflammation of the thyroid gland (thyroiditis), and taking too much thyroid replacement hormone to treat an underactive thyroid (hypothyroidism).

Hyperthyroidism can be treated with medication, radioactive iodine, or with surgery. In most cases, treatment brings thyroid function back to normal. However, lifelong follow-up with a health professional is needed to closely monitor thyroid hormone levels.