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

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Abstract
Nonthyroidal illness is frequently associated with subnormal serum thyroxine (T4) and free T4 index. To unravel the resultant diagnostic problems, several variables of thyroid function in the sera of 47 patients hospitalized with nonthyroidal illnesses and 7 hypothyroid patients encountered during the same period were studied. Of the 47 euthyroid sick patients, 18 had low T4. Among these 18, free T4 index was normal in only 5, whereas free T4 concentration measured by equilibrium dialysis was normal or high in 15 and reverse triiodothyronine (rT3) normal or high in all 18. rT3, free T4 concentration and free T4 index were subnormal in all 7 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 rT3 and free T4 is quite discriminating.