Meaning of serum free-thyroxin values in nonthyroidal illnesses: seven methods compared

Serum free thyroxin (FT4) was determined in 40 patients with various nonthyroidal illnesses. We studied seven methods: (1) a free thyroxin index calculated from total T4 and triiodothyronine resin uptake; (2) a free T4 index determined by enzyme inhibitor assays (Abbott's "Tetrazyme" and "Thyrozyme"); (3) a free T4 index calculated from total T4 and thyroxin-binding globulin; (4) free T4 by equilibrium dialysis; (5) Amersham's free T4 RIA; (6) Clinical Assays' one-step free T4 RIA; and (7) Clinical Assays' two-step free T4 RIA. Approximately half of the free T4 results were in the euthyroid range and the other half in the hypothyroid range by methods 1, 2, 5, and 6. Results for free T4 by methods 3 and 7 were similar to those by equilibrium dialysis (method 4), the percentages of patients with results in the euthyroid range being 68%, 65%, and 76%, respectively.