SUMMARYThe effective thyroxine ratio (ETR), a single test of thyroid function which has a highly significant linear correlation with serum free thyroxine (T4) concentration, was assessed for its effectiveness in distinguishing the euthyroid state in subjects with abnormalities of T4‐binding proteins.In patients with low activity of T4‐binding proteins, the diagnostic accuracy of the ETR (80±5%) is superior to that of the free T4index (62±5%) and free T4concentration (67%). In patients with elevated T4‐binding proteins the diagnostic accuracy of ETR was even higher at 93±5% and again better than the free T4index (83%) and the free T4concentrati