There are, then, two "vicious circles," or feedback loops, that can play a role in causing or aggravating emotional disturbance: [Illustration: The two "vicious circle" or feedback loops that may exist between physical disorders and emotional disturbances] In the first loop, an underlying physical disorder, which may be a disease or a physical injury, leads to emotional symptoms (and very likely to physical symptoms as well, although these may not be as pronounced). However, the emotional reactions that are produced can themselves make the physical disorder worse, and certainly emotional disturbance makes living with and treating the underlying physical disorder more difficult. In the second loop, emotional disturbances cause certain physical disorders: a peptic ulcer, heart palpitations, ulcerative colitis, backache, hypertension, or high blood pressure, etc., and may predispose certain individuals to arthritis, cancer, or diseases of the immune system. Once a psychosomatic link has been established between a troubled mind and the body, and an organic disorder has come about, the physical disorder, in turn, can produce stronger or more exaggerated emotional reactions. Anxiety or depression may increase because the person is now both physically ill and emotionally troubled. These two so-called positive feedback loops can obviously {102} lead to a runaway process that becomes worse and worse. Psychosomatic medicine focuses on the second of these; our focus here is on the first: physical origins of emotional disturbance. Underlying physical disorders of this kind include metabolic diseases, disorders and diseases affecting the brain and nervous system, head injuries, other physical disorders and conditions, infectious diseases, reactions to medication, and drug addiction and alcoholism. Each is discussed in the remainder of this chapter. METABOLIC DISEASES Several well-known metabolic diseases can lead to emotional disturbances: HYPERTHYROIDISM An overactive thyroid, known as _hyperthyroidism_, is usually caused by the pituitary gland's overproduction of a hormone called _TSH_, or _thyroid-stimulating hormone_. This causes the thyroid, a butterfly-shaped gland in the lower part of the neck, to produce an excess of the thyroid hormone thyroxine. Hyperthyroidism is eight times more common in women than in men. The emotional symptoms of hyperthyroidism include a more intense and chronic nervousness than in hypothyroidism (discussed below), overreactions to minor crises, moodiness, frequent fear without knowing why, a sense of agitation, dread, and occasionally trembling or shaking. Some patients with serious hyperthyroidism may have symptoms resembling those of schizophrenia, in which there is little or no contact with reality.