Apathetic hyperthyroidism refers to hyperthyroidism without clear symptoms of thyrotoxicosis (see Chapter 4). Normally, people who are thyrotoxic as a result of hyperthy-roidism have a racing heart, enlarged thyroid gland, tremor, restlessness and anxiety, and signs of thyroid eye disease (see Chapter 23). In apathetic hyperthyroidism, the telltale signs doctors count on to diagnose hyperthyroidism are absent; instead, a person with apathetic hyperthyroidism may even seem calm and mild-mannered. Or, the person could exhibit more hypothyroid symptoms, such as depression and constipation. Typically, the only sign of hyperthyroidism in an older person is weight loss, fatigue, and irritability, which are easily attributed in most cases to normal aging. In other instances, heart rhythm disturbances (usually rapid heartbeats or atrial fibrillation) are the only clues to thyrotoxicosis. In short, many people with apathetic hyperthyroidism are missed, and the hyperthyroidism remains untreated and unrecognized. The only objective indicator of hyperthyroidism is a low TSH test result and a high free T4 test result (see Chapter 2). In people with low TSH tests but normal T4 levels, the term subclinical hyperthyroidism is used, which means that they have mild hyperthy-roidism that is not yet resulting in obvious symptoms.
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