Hyperthyroidism is a disorder that develops when some or all of the tissue that makes up the thyroid gland becomes overactive, producing and secreting excess amounts of thyroid hormone. The word "hyperthyroidism" is often used to describe any disorder associated with the accumulation of excess amounts of thyroid hormone. Some disorders associated with the accumulation of excess amounts of thyroid hormone may actually be associated with decreased activity of thyroid tissue. It is more correct to refer to these disorders as different forms of thyrotoxicosis. Patients who are diagnosed with hyperthyroidism represent a subset of all patients diagnosed with thyrotoxicosis. The distinction is subtle, but it may be important with respect to the selection of different treatments. Medications and treatments that work to suppress overactive thyroid tissue may not be effective in the treatment of disorders associated with decreased activity of thyroid tissue.
True overactivity of thyroid tissue may be caused by different mechanisms. In the setting of certain autoimmune disorders, antibodies produced by the body's immune system may bind to structures on the outer surface of follicular cells called TSH receptors. This may stimulate increased production and secretion of thyroid hormone. In the setting of disorders associated with abnormal growth of thyroid tissue, clusters of follicular cells may escape from the normal control exerted by the pituitary gland. This may lead to uncontrolled production and secretion of thyroid hormone. As these clusters of follicular cells expand, they may form nodules within the lobes of the thyroid gland that may be detected on physical examination.