In response to Dr Wiener's letter, we have reexamined our data by calculating a "free T3 index," T3 level divided by unsaturated thyroid hormone binding capacity index. From the 32 normal subjects, a range of 77 to 165 ng/dL was determined. From the seven patients with elevated T3 and normal T4 levels, a range of 209 to 348 ng/dL was found. Thus, in this particular group, calculation of a free T3 index did not alter the impression that notable T3 elevation was present, despite the fact that some of the patients failed to show the suppressed levels of TSH that we find associated with hyperthyroidism. Our conclusion remains unchanged, namely, that among individuals with high T3 levels and normal T4 levels, only certain of them merit the diagnosis of "T3 toxicosis."
Our data are at variance with Dr Wiener's implied suggestion that TSH secretion is controlled largely by free T3.
Smith SR. Serum Thyroid-Stimulating Hormone Values-Reply. Arch Intern Med. 1980;140(9):1251-1252. doi:10.1001/archinte.1980.00330200127044