Final answer:
Graves' disease is the most common cause of hyperthyroidism, an autoimmune condition that leads to an overproduction of thyroid hormones, often presenting with symptoms such as heat intolerance and rapid heartbeat.
Step-by-step explanation:
Hyperthyroidism, characterized by an overproduction of thyroid hormones, is most commonly caused by an autoimmune condition known as Graves' disease. This condition involves the production of thyroid-stimulating immunoglobulin (TSI), which acts upon the thyroid gland. TSI can either stimulate the thyroid to produce excessive amounts of hormone or block its production entirely. Symptoms of Graves' disease may include heat intolerance, rapid and irregular heartbeat, weight loss, goiter, and exophthalmos, which are among the typical presentations of this hyperthyroid state.
The answer to the student's question is (d. Graves' disease), as it is the most common cause of hyperthyroidism due to an autoimmune reaction that causes overstimulation of the thyroid gland's follicle cells.