Diagnostic Value of Serum Thyroxine and Free Thyroxine Index

A modified method for determination of serum total thyroxine (T 4 ) using competitive protein‐binding is reported. This method proved particularly valuable when analysing a large number of serum samples. A clinical evaluation of T 4 was performed in various thyroid disorders. T 4 was found to be superior to serum protein‐bound iodine (PBI) in hyperthyroidism, mainly because of frequently encountered spuriously increased PBI values. In hypothyroidism PBI proved to be more accurate than T 4 , and there were many patients with low‐normal T 4 values who benefited by administration of thyroid hormones. Three different free thyroxine indices were studied. One was similar to the original free thyroxine index, here abbreviated T 8 I, defined as the product of PBI and triiodothyronine uptake by Sephadex (T 3 U). Another free thyroxine index (FT 4 I) was calculated as the product of T 4 and thyroxine uptake by Sephadex (T 4 U). The product of T 4 and T a U was the third free thyroxine index (FT 3 I) studied. Of these indices T 3 I had the lowest diagnostic accuracy in both hyper‐ and hypothyroidism. FT 4 I and FT 3 I were of approximately equal value in hyper‐ and hypothyroidism. The clinical usefulness of both FT 4 I and FT 3 I was superior to that of T 4 in all conditions studied. It is recommended that both of these free thyroxine indices are determined when mild hyper‐ or hypothyroidism is suspected.