The determination of circulating levels of thyroid hormones is essential for an accurate assessment of the functional status of patients. By means of specific competitive immunoassays (radioimmunoassays [RIAs], enzyme or chemiluminescent immunoassays [EIAs; CLIAs], or immunoradiometric assays
[IRMAs]), small quantities of these and other hormones can be detected, enabling physicians to diagnose even mild degrees of thyroid dysfunction. RIA methodology involves a reaction between a specific antibody and two antigens, one of which is radiolabeled (see Chap. 237). In addition, various other laboratory procedures are available for examining thyroid disorders.

Numerous factors may affect the function of the hypothalamic–pituitary–thyroid axis. These include physiologic factors (age, pregnancy, stress, temperature, genetics), pathologic factors (hyper- or hypothyroidism, systemic illness, surgery, starvation), and pharmacologic factors (Table 33-1). Some of these conditions produce a bewildering array of alterations in serum concentrations of these hormones, and a carefully selected set of tests may be required to establish the correct diagnosis. These tests are discussed in this chapter.