SUMMARY The effective thyroxine ratio (ETR), a single test of thyroid function which has a highly significant linear correlation with serum free thyroxine (T 4 ) concentration, was assessed for its effectiveness in distinguishing the euthyroid state in subjects with abnormalities of T 4 ‐binding proteins. In patients with low activity of T 4 ‐binding proteins, the diagnostic accuracy of the ETR (80±5%) is superior to that of the free T 4 index (62±5%) and free T 4 concentration (67%). In patients with elevated T 4 ‐binding proteins the diagnostic accuracy of ETR was even higher at 93±5% and again better than the free T 4 index (83%) and the free T 4 concentration (91%).
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Wellby et al. (1974) studied this question.
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