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March 5, 1973

Normalized Serum Thyroxine and Effective Thyroxine Ratio

Author Affiliations

Miami, Fla

JAMA. 1973;223(10):1160. doi:10.1001/jama.1973.03220100054026

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To the Editor.—  In a recent letter (222:1653, 1972) Mincey compared normalized thyroxine (T4N) with the effective thyroxine ratio (ETR) by two in vitro thyroid-function tests utilizing modified competitive techniques of protein binding analysis. He claimed that the techniques were the same and that we failed to mention his original work (Clin Biochem 4:216, 1971).The T4N and ETR tests are different in principle, method, technique, and results. The T4N test is a superior test because it is a quantitative measurement of serum thyroxine; it does not alter normal or pathological thyroxine values and corrects falseabnormal thyroxine values due to the interference of certain physiological conditions. It possesses good separation between the normal and abnormal thyroid functional state, resulting in no overlap between them.The ETR test is an indirect measure of thyroxine as an index, is not quantitative, and has a very narrow normal