<h3>To the Editor.</h3> —The American Thyroid Association (ATA) in 1990 and 1991 issued guidelines for use of laboratory tests in thyroid disorders.<sup>1,2</sup>These ATA position papers update previous recommendations because of the development and availability of improved thyrotropin (thyroid-stimulating hormone [TSH]) methods capable of clearly separating normal from subnormal serum TSH levels. The highly sensitive TSH assay constitutes a practical, useful, and significant laboratory advance in clinical thyroidology. There is agreement among thyroid specialists that measurement of serum TSH level, complemented by an appropriate free thyroxine (FT<sub>4</sub>) estimate, represents the best and most efficient combination of blood tests for diagnosis and follow-up of most patients with thyroid disorders. Currently in the investigation of thyroid disease there is a trend to replace a combination of tests (eg, total T<sub>4</sub>+ triiodothyronine resin uptake + FT<sub>4</sub>+ total T<sub>3</sub>+ TSH) by a discriminating single thyroid function test. This trend reflects