Laboratory Testing

Tests for thyroid disorders include laboratory, imaging, and biopsy. Before imaging or biopsy is undertaken, it is important to determine the functional state of the thyroid gland, even when the initial presentation is a thyroid mass or thy-romegaly. This is accomplished via laboratory testing of a peripheral blood sample. These simple, and readily available, tests will provide direction for further workup.

The initial laboratory tests, regardless of the presenting complaint or finding, include sTSH and FT4. The results of these initial studies help determine the functional state of the gland (hyperthyroid, euthyroid, or hypothyroid) and thus suggest which additional tests are required (Table 35-1).

Second-tier laboratory tests include thyroid antibodies and FT3 if T3 toxicosis is suspected. As noted previously, aging and comorbidities that affect circulating levels of thyroid transport protein can result in T4 levels that appear abnormally low, suggesting a hypothyroid state. However, FT4 and FT3 will be normal, as will sTSH. Thyroid antibodies are useful in evaluating several disease states, primarily Graves' disease and chronic autoimmune thyroiditis (CAT, Hashimoto's thyroiditis). In Graves' disease the primary antibody class is TSH receptor-stimulator antibodies (TSH-RS Abs). In CAT the primary antibodies are thyroid antiper-oxidase antibodies (TPO Abs) and thyroglobulin antibodies (Tg Abs). Patients with hypothyroidism occasionally exhibit TSH receptor-blocker antibodies (TSH-RB Abs), although the role this plays in the disease course is unclear. Thyroid microsomal antibodies (TPO Abs, Tg Abs) are occasionally seen in the self-limited processes of postpartum thyroiditis and silent thyroiditis.

Figure 35-1 provides an algorithm for diagnosing thyroid dysfunction.

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