Effectiveness of analysis of free thyroxin concentration in serum for diagnosis of covert thyroid disease.

The indirect indices of free-thyroxin concentration (fT4) are of limited effectiveness in diagnosing covert (mild) thyroid disease. We compared measurements of fT4 and of normalized T4 (NT4, the effective thyroxin ratio) in a group of patients being treated for thyroid disease, most of whom were clinically euthyroid. Although fT4 values correlated well with NT4, many of the patients (34% in one of the comparisons) had fT4 and NT4 values that gave different indications of thyroid status. A review of the patients' treatment and other biochemical indications revealed that measurements of fT4 were more effective in detecting mild disturbances of thyrometabolic status than were assays for NT4, total T4, total triiodothyronine, or thyrotropin. This study suggests that direct fT4 assays are particularly useful in diagnosing covert thyroid disease.