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Sensitive thyroid-stimulating hormone assays: clinical applications and limitations.

Sensitive TSH assays have important applications in various conditions, including the diagnosis of hypothyroidism and hyperthyroidism, monitoring thyroid hormone therapy and treated thyrotoxic patients, and evaluating thyroid dysfunction in nonthyroidal illnesses and pregnancy. Interpretation of the TSH value should be made with a clear understanding of its limitations. TSH may be inappropriately secreted by pituitary tumors and by pituitary dysfunction due to thyroxine resistance. At present, it is uncertain whether clinically euthyroid patients with autonomously functioning thyroid nodules, or with multinodular goiters, or patients taking thyroid hormone who have suppressed TSH values, are actually euthyroid at a cellular level. Other factors that affect TSH levels are the biologic variation in its secretion, the presence of heterophilic antibodies in a patient's serum, and various drugs. But perhaps the most important factor affecting the TSH assay is severe nonthyroidal illness in hospitalized patients. The new ultrasensitive TSH assay does not yet replace other thyroid function tests, but it is clearly emerging as an important means of screening patients for thyroid dysfunction, especially ambulatory patients without other serious illnesses. It can usually separate patients with thyroid dysfunction from euthyroid individuals. Good clinical assessment is always necessary, and other thyroid function tests are often needed. The sensitivity of these new TSH assays in the diagnosis of thyrotoxicosis and hypothyroidism is excellent; the specificity is not as good. Nonetheless, at present this test can be used in the initial diagnosis of thyroid dysfunction as outlined in Figure 2.(ABSTRACT TRUNCATED AT 250 WORDS)