Abstract
To the Editor. —In the July 1979 issue of theArchives(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 (T 3 ) levels but normal thyroxine (T 4 ) levels, however, TSH values were sometimes midnormal. These results were thought to indicate heterogeneity within the latter group, only some of these patients having 3 toxicosis. Now it is known that both serum T 4 and T 3 levels may be relatively high in the presence of elevated T 4 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 T 3 than by total T 3 . 1 A free T , index can easily be calculated from total T3 level and a
Published Version
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