Thyrotoxicosis of other Etiologies

Thyrotoxicosis is defined as the clinical syndrome of hypermetabolism resulting from increased free thyroxine (T4) and/or free triiodothyronine (T3) serum levels (1). The term thyrotoxicosis is not synonymous with hyperthyroidism, the elevation in thyroid hormone levels caused by an increase in their biosynthesis and secretion by the thyroid gland (Table 1) (2). For example, thyrotoxicosis can result from the destruction of thyroid follicles and thyrocytes in the various forms of thyroiditis, or it can be caused by an excessive intake of exogenous thyroid hormone. It should also be noted that the elevation of free thyroid hormone levels does not always result in thyrotoxicosis in all tissues. In the syndrome of Resistance to Thyroid Hormone (RTH), dominant negative mutations in the thyroid hormone receptor  ( TR ) result in decreased thyroid hormone action in tissues where TR is the predominant receptor, for example in the liver and the pituitary, whereas other tissues such as the heart, which express mainly TR, show signs of increased thyroid hormone action (See Chapter 16 D). The most common form of thyrotoxicosis is Graves’ disease, which is discussed in Chapter 10. This chapter reviews other etiologies of thyrotoxicosis (Table 1). The determination of the etiology of thyrotoxicosis is of importance in order to establish a rational therapy.