A nomenclature for tests of thyroid hormones in serum: report of a committee of the American Thyroid Association.

Abstract

For many years, the serum protein-bound iodine (PBI) test was the only available measurement to estimate the concentration of thyroid hormone in serum. In recent years, a succession of new tests has appeared, allowing improved diagnosis of the state of thyroid function. However, the very multiplicity of tests has resulted in a confusing welter of names, at times ambiguous to the point of paralysis of effective communication. For example, thyroxine (T4) iodine measured by column chromatography has regularly been reported as “serum thyroxine-by-column,” whereas it actually measures only 65 percent of the T4 molecule (i.e., its four iodines) and thus, has a much lower normal range than serum T4 and, further, is susceptible to artefactual elevation by iodinated compounds entirely different from the thyroid hormones. In clinics and at meetings, physicians speak of “the T4 test,” although T4 iodine may actually have been measured. They speak also of “the T3 test,” when the resin uptake of labeled T3 is meant, a serious source of confusion now that estimation of the concentration of nonradioactive T3 in serum is approaching practical applicability (1–4) and appears to have real clinical utility (5). Meanwhile, with the proposal of the “free thyroxine index” by Clark and Horn (6), a new dimension of hormone measurement was added.