What is the most common cause of hyperthyroidism?

Graves disease is the most common cause of hyperthyroidism because it is an autoimmune disorder where the immune system mistakenly stimulates the thyroid gland to produce excess thyroid hormone. This overproduction leads to the clinical manifestations of hyperthyroidism, such as elevated heart rate, weight loss, anxiety, and heat intolerance, as well as physical signs like goiter and eye changes (Graves' ophthalmopathy).

The other conditions listed contribute less frequently to hyperthyroidism. Thyroid nodules can lead to hyperthyroidism when they secrete thyroid hormones independently, but this scenario is not as prevalent as the autoimmune response seen in Graves disease. Hashimoto thyroiditis typically causes hypothyroidism, although in some cases, it may briefly cause hyperthyroidism due to inflammation and release of stored thyroid hormones. Cushing's syndrome is primarily related to excess cortisol rather than thyroid hormone, making it an unlikely cause of hyperthyroidism. Thus, Graves disease stands out as the primary and most common driver of hyperthyroid conditions.