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Misleadingly Low Free Thyroxine Index and Usefulness of Reverse Triiodothyronine Measurement in Nonthyroidal Illnesses

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This study was supported in part by U.S. Public Health Service grants AM-16155 and AM17251.

▸Requests for reprints should be addressed to Inder J. Chopra, M.D.; Department of Medicine, UCLA Center for the Health Sciences; Los Angeles, CA 90024.

Los Angeles and Torrance, California

© 1979 American College of PhysiciansAmerican College of Physicians

Ann Intern Med. 1979;90(6):905-912. doi:10.7326/0003-4819-90-6-905
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Nonthyroidal illness is frequently associated with subnormal serum thyroxine (T4) and free T4 index. To unravel the resultant diagnostic problems, we have studied several variables of thyroid function in the sera of 47 patients hospitalized with nonthyroidal illnesses and seven hypothyroid patients encountered during the same period. Of the 47 euthyroid sick patients, 18 had low T4. Among these 18, free T4 index was normal in only five, whereas free T4 concentration measured by equilibrium dialysis was normal or high in 15 and 3,3',5'-triiodothyronine (reverse T3) normal or high in all 18. Reverse T3, free T4 concentration, and free T4 index were subnormal in all seven hypothyroid patients. Thus, measurement of free T4 index may be misleading in evaluation of thyroid function in patients with nonthyroidal illnesses, whereas measurement of serum concentration of reverse T3 and free T4 is quite discriminating.





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