GOITER AND NODULAR THYROID DISEASE
Goiter refers to an enlarged thyroid gland. Biosynthetic defects, iodine deficiency, autoimmune disease, and nodular diseases can each lead to goiter, although by different mechanisms. Biosynthetic defects and iodine deficiency are associated with reduced efficiency of thyroid hormone synthesis, leading to increased thyroid-stimulating hormone (TSH), which stimulates thyroid growth as a compensatory mechanism to overcome the block in hormone synthesis. Graves’ disease and Hashimoto’s thyroiditis are also associated with goiter. In Graves’ disease, the goiter results mainly from the TSH-R–mediated effects of thyroid-stimulating immunoglobulins. The goitrous form of Hashimoto’s thyroiditis occurs because of acquired defects in hormone synthesis, leading to elevated levels of TSH and its consequent growth effects. Lymphocytic infiltration and immune system–induced growth factors also contribute to thyroid enlargement in Hashimoto’s thyroiditis.
Thyroid nodular disease is characterized by the disordered growth of thyroid cells, which can be either hyperplastic or neoplastic. A patient may have a multinodular goiter (MNG) in which thyroid nodules (generally hyperplastic) replace the majority of the normal thyroid parenchyma; this presentation is more common in areas of borderline iodine deficiency. Or, the thyroid gland may be normal in size and contain discrete thyroid nodules. Because the management of goiter depends on the etiology, the detection of thyroid enlargement on physical examination should prompt further evaluation to identify its cause.
Nodular thyroid disease is common, occurring in about 3–7% of adults when assessed by physical examination. Using ultrasound, nodules are present in up to 50% of adults, with the majority being <1 cm in diameter. Thyroid nodules may be solitary or multiple, and they may be functional or nonfunctional.
DIFFUSE NONTOXIC (SIMPLE) GOITER
Etiology and Pathogenesis
When diffuse enlargement of the thyroid occurs in the absence of nodules and hyperthyroidism, it is referred to as a diffuse nontoxic goiter. This is sometimes called simple goiter, because of the absence of nodules, or colloid goiter, because of the presence of uniform follicles that are filled with colloid. Worldwide, diffuse goiter is most commonly caused by iodine deficiency and is termed endemic goiter when it affects >5% of the population. In nonendemic regions, sporadic goiter occurs, and the cause is usually unknown. Thyroid enlargement in teenagers is sometimes referred to as juvenile goiter. In general, goiter is more common in women than men, probably because of the greater prevalence of underlying autoimmune disease and the increased iodine demands associated with pregnancy.
In iodine-deficient areas, thyroid enlargement reflects a compensatory effort to trap iodide and produce sufficient hormone under conditions in which hormone synthesis is relatively inefficient. Somewhat surprisingly, TSH levels are usually normal or only slightly increased, suggesting increased sensitivity to TSH or activation of other pathways that lead to thyroid growth. Iodide appears to have direct actions on thyroid vasculature and may indirectly affect growth through vasoactive substances such ...