Hyperthyroidism is the condition that occurs due to excessive production of thyroid hormones by the thyroid gland. Thyrotoxicosis is the condition that occurs due to excessive thyroid hormone of any cause and therefore includes hyperthyroidism. Some, however, use the terms interchangeably. Signs and symptoms vary between people and may include irritability, muscle weakness, sleeping problems, a fast heartbeat, heat intolerance, diarrhea, enlargement of the thyroid, hand tremor, and weight loss. Symptoms are typically less severe in the elderly and during pregnancy. An uncommon but life-threatening complication is thyroid storm in which an event such as an infection results in worsening symptoms such as confusion and a high temperature; this often results in death. The opposite is hypothyroidism, when the thyroid gland does not make enough thyroid hormone.

Hyperthyroidism
Other names	Overactive thyroid, hyperthyreosis
Triiodothyronine (T3, pictured) and thyroxine (T4) are both forms of thyroid hormone.
Specialty	Endocrinology
Symptoms	Irritability, muscle weakness, sleeping problems, fast heartbeat, heat intolerance, diarrhea, enlargement of the thyroid, weight loss
Complications	Thyroid storm
Usual onset	20–50 years old
Causes	Graves' disease, multinodular goiter, toxic adenoma, inflammation of the thyroid, eating too much iodine, too much synthetic thyroid hormone
Diagnostic method	Based on symptoms and confirmed by blood tests
Treatment	Radioiodine therapy, medications, thyroid surgery
Medication	Beta blockers, methimazole
Frequency	1.2% (US)
Deaths	Rare directly, unless thyroid storm occurs; associated with increased mortality if untreated (1.23 HR)

Graves' disease is the cause of about 50% to 80% of the cases of hyperthyroidism in the United States. Other causes include multinodular goiter, toxic adenoma, inflammation of the thyroid, eating too much iodine, and too much synthetic thyroid hormone. A less common cause is a pituitary adenoma. The diagnosis may be suspected based on signs and symptoms and then confirmed with blood tests. Typically blood tests show a low thyroid stimulating hormone (TSH) and raised T₃ or T₄. Radioiodine uptake by the thyroid, thyroid scan, and measurement of antithyroid autoantibodies (thyroidal thyrotropin receptor antibodies are positive in Graves disease) may help determine the cause.

Treatment depends partly on the cause and severity of disease. There are three main treatment options: radioiodine therapy, medications, and thyroid surgery. Radioiodine therapy involves taking iodine-131 by mouth which is then concentrated in and destroys the thyroid over weeks to months. The resulting hypothyroidism is treated with synthetic thyroid hormone. Medications such as beta blockers may control the symptoms, and anti-thyroid medications such as methimazole may temporarily help people while other treatments are having an effect. Surgery to remove the thyroid is another option. This may be used in those with very large thyroids or when cancer is a concern. In the United States hyperthyroidism affects about 1.2% of the population. Worldwide, hyperthyroidism affects 2.5% of adults. It occurs between two and ten times more often in women. Onset is commonly between 20 and 50 years of age. Overall the disease is more common in those over the age of 60 years.