To the Editor.
—In the July 1979 issue of the Archives (139:757-760) Smith et al report an interesting observation. With a very sensitive test for thyroid-stimulating hormone (TSH) in serum, they found the TSH level to be nearly always very low in unequivocal hyperthyroidism; in patients with elevated triiodothyronine (T3) levels but normal thyroxine (T4) levels, however, TSH values were sometimes midnormal. These results were thought to indicate heterogeneity within the latter group, only some of these patients having "T3 toxicosis."Now it is known that both serum T4 and T3 levels may be relatively high in the presence of elevated T4 binding capacity (eg, in pregnancy and many oral contraceptive users). When this category is not excluded, euthyroidism and hyperthyroidism are better separated by a measure of free T3 than by total T3.1 A free T, index can easily be calculated from total T3 level and a