Neoplasia of a tissue occasionally results in the production of a substance with hormonal activity similar to that produced by one of the endocrine glands. This leads to syndromes that may simulate hyperfunction of an endocrine gland. The syndromes that have been well documented are: hypoglycemia, Cushing's syndrome, polycythemia, hypercalcemia, precocious puberty, the atypical carcinoid syndrome, inappropriate secretion of an anti-diuretic substance, and hyperthyroidism. In Cushing's syndrome and polycythemia, the substances produced have many of the physicochemical characteristics of the normal hormones, adrenocorticotropic hormone and erythropoietin respectively. However, the syndromes of hypoglycemia and hyperthyroidism are clearly not due to the production of insulin or thyroid-stimulating hormone but rather to substances that have these biologic activities. Recognition of these syndromes will result in adequate medical management and occasionally cure.

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