Key Points

• Primary thyroid disorder is caused by abnormal function of the thyroid gland.

• Secondary thyroid disorder is the result of abnormalities at the level of the pituitary.

• Tertiary thyroid disorder results from malfunction at the level of the hypothalamus.

Thyroid disorders include processes that affect function (physiology) as well as structure (anatomy). Extraglandular causes include metastatic neoplasia, pituitary disorders, dietary issues, autoimmune diseases, infections, and genetic or familial diseases, such as multiple endocrine neoplasia IIA and familial medullary thyroid carcinoma. Other causes are intrinsic to the thyroid and include cysts, nodules, and goiter. In either case, all thyroid diseases exist in one of three functional states: euthyroid, hyperthyroid, or hypothyroid; each is defined by the level of total bound and free, circulating thyroid hormone. The presence of any one of these states in an individual can be transient, static, or progressing. Laboratory abnormalities of circulating thyroid hormone, at any point in time, do not prove disease and do not depend on the etiology of thyroid dysfunction. All three states may exist at different times during the course of an illness, and each state can exist with or without disease or clinical findings. In addition, the various thyroid structures can reflect disease independent of endocrinologic function.

Accurate assessment of thyroid function, with determination of presence or absence of disease, requires data in addition to levels of circulating thyroid hormones. These data include serum free and total thyroid hormone levels, thyrotropin (TSH) levels, and in some cases, antithyroid antibody (ATA) titers. This battery of tests will provide diagnosis in the majority of common thyroid disorders. When imaging studies and fine-needle aspiration are added, 90% to 95% of patients with thyroid disease who present in the primary care setting can be diagnosed and appropriately managed.

Thyroid disorders affect 60 to 80 per 1000 adults worldwide and up to 8.9% of the adult U.S. population (Bagchi et al., 1990; Vanderpump et al., 1995). Since most have an insidious onset or closely mimic other, more common disorders, thyroid disorders are easily missed and, although rarely fatal, can cause significant morbidity. Early recognition is critical to minimizing morbidity. With the exception of conditions such as simple goiter or visible nodule, patients who ultimately are

Histologically, the thyroid gland consists of five primary elements: follicular cells, colloid, interstitial tissue, "C" cells, and lymphoid cells. The most prominent element is the fol-licular cell, which produces colloid. The thyroid follicle is the functional unit of the gland and the site where colloid is stored. It is within the follicle where thyroid hormone (thyroxine, or T4) synthesis occurs. The remaining cellular elements are C cells and lymphoid cells. The few C cells are located in the intrafollicular space and produce calcitonin. Lymphoid cells are found scattered throughout the gland stroma in small, isolated clusters.

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