Condition in which thyroid hormones are overproduced. It is one of the most common hormone disorders, occurring five times as often in women as men. Under normal circumstances, thyroid function is regulated by the pituitary gland, which produces substances that stimulate the thyroid to release hormones into the blood. In hyperthyroidism this regulatory mechanism is disturbed, and the thyroid begins to produce large quantities of hormones unecessarily. Sometimes this causes the gland to swell (goitre), in other cases it is only partially active (thyroid tumour). The most important task of thyroid hormones is the acceleration of a number of metabolic processes; overproduction of hormones means excessive acceleration. Mentally this results in agitation, irritability and tiredness, but also sleeplessness. Because the metabolic processes release heat, the patient is too hot even in cold weather, and sweats coninuously. Other symptoms are trembling hands, and a rapid, sometimes irregular, heartbeat. An accelerated metabolism also causes increased appetite, because a great deal of energy is needed, but despite this the patient loses weight. This is partly a result of diarrhoea, because the intestines process food more rapidly; in women menstruation is often irregular or completely absent. Certain forms of hyperthyroidism also produce eye abnormalities (in Graves’ disease) such as exopthalmos, and sometimes blurred or double vision. Because heart and blood vessels are overloaded there is a danger of angina pectoris or heart failure, particularly in older people. Hyperthyroidism is diagnosed by checking blood hormone levels. Treatment is by medication or radioactive iodine therapy and surgery. In half the cases the use of drugs to restrict thyroid hormone formation seems effective within six months. If radioactive iodine is injected it accumulates in the thyroid and disables the cells responsible for overproduction. Surgery is used to remove part of the thyroid, in the hope that the remainder will function normally. All the treatments have some advantages and some disadvantages; medication, for example, can sometimes cause severe blood disorders (agranulocytosis), and there is also the danger that thyroid production will be insufficiently or unduly limited. In the latter case extra thyroid hormones have to be administered, sometimes for the rest of the patient’s life.