The classic picture of hyperthyroidism or hypothyroidism is frequently not complete and may be totally absent. There may be only one noticeable symptom or sign, and even that may be vague or misleading or suggestive of some other disease. The physician must turn to the laboratory to confirm or exclude the diagnosis.

There is a comparatively large group of laboratory procedures that measure one or more aspects of thyroid function (see the box). The multiplicity of these tests implies that none is infallible or invariably helpful. To get best results, one must have a thorough knowledge of each procedure, including what aspect of thyroid function is measured, sensitivity of the test in the detection of thyroid conditions, and the rate of false results caused by non thyroid conditions. In certain cases, a brief outline of the technique involved when the test is actually performed helps clarify some of these points.