Hyperthyroidism (thyrotoxicosis) is a syndrome resulting from a chronic excess of thyroid hormone (thyroxine). It may be precipitated by acute emotional stress and may even develop within hours after an emotional trauma. It is an endocrine disorder and is almost always accompanied by mental changes—the patient may feel tense and hyperexcitable and may be emotionally labile, with inappropriate temper outbursts, crying spells, or euphoria. Distractibility, short attention span, and impaired recent memory may also be present. Severe hyperthyroidism may result in frank psychosis, delirium, coma, and death. Thyroid disorders are 7 times more frequent in women than in men; they occur most commonly in women in their third and fourth decades and in men at older ages. In contrast to the tense and hyperexcitable behaviors usually observed, a minority of patients, particularly the elderly, may be depressed, apathetic, and anorectic with chronic hyperthyroidism. Mildly hyperthyroid patients occasionally are misdiagnosed as having anxiety neuroses. Mental retardation in children is frequently associated with hypothyroidism, not hyperthyroidism, since normal brain development requires the thyroid hormone.