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.