What is the main cause of hyperthyroidism?

Hyperthyroidism is predominantly caused by Graves' disease, an autoimmune disorder characterized by the overproduction of thyroid hormones due to the stimulation of the thyroid gland. In Graves’ disease, antibodies are produced that mimic thyroid-stimulating hormone (TSH), leading to excessive release of hormones like thyroxine (T4) and triiodothyronine (T3). This results in the symptoms associated with hyperthyroidism, such as increased metabolic rate, weight loss, and anxiety.

While thyroid cancer can cause hyperthyroidism in rare cases, it is not a common cause. Hashimoto's disease, on the other hand, typically leads to hypothyroidism due to the autoimmune destruction of the thyroid gland, rather than hyperthyroidism. An adrenal adenoma affects adrenal glands and is unrelated to the direct functioning of the thyroid gland. Therefore, Graves' disease is the primary and most prevalent cause of hyperthyroidism, making it the correct answer in this context.