A goiter is an enlarged thyroid gland, which causes a pronounced bulge on the front and sides of the neck. Goiter can be associated with either hyperthyroidism (very high levels of thyroxine) or hypothyroidism (very low levels of thy-roxine). The negative feedback loop whereby thyroxine controls TSH release helps explain how two very different conditions can result in the same symptom, but it is also necessary to understand how the thyroid makes, stores, and releases thyroxine.
Each thyroid follicle consists of a layer of epithelial cells surrounding a mass of glycoprotein called thyroglobulin. The thyroglobulin, which consists of many residues of tyrosine, is made by the epithelial cells . The tyrosine residues are io-dinated as the thyroglobulin is secreted into the center of the follicle. When thyroxine is needed, the same epithelial cells that made the thyroglobulin take it back and digest it to release thyroxine molecules. If there was enough iodine available when the thyroglobulin was made, its digestion releases molecules of T3 and T4. If there was not enough iodine available when the thyroglobulin was made, many of the residues released will not be T3 or T4 and will not bind to receptors on target cells.
Goiter occurs when the production of thyroglobulin is far above normal and the follicles become greatly enlarged. Hy-perthyroid goiter results when the negative feedback mechanism fails to turn off the follicle cells even though blood levels of thyroxine are high. The most common cause of hyperthyroidism is an autoimmune disease in which an antibody to the TSH receptor is produced. This antibody can bind to the TSH receptor on the follicle cells, causing them to produce and release thyroxine. Even though blood levels of TSH may be quite low because of the negative feedback from high levels of thyroxine, the thyroid remains maximally stimulated, and it grows bigger. Hyperthyroid patients have high metabolic rates, are jumpy and nervous, usually feel hot, and may have a buildup of fat behind the eyeballs, causing their eyes to bulge.
Hypothyroid goiter results when there is not enough circulating thyroxine to turn off TSH production. The most
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