A reappraisal of the free thyroxine index.

The usefulness of the routine measurement of the free thyroxine index as opposed to that of total thyroxine only has been examined by analysis of 803 requests for thyroid function tests in adults. In only 2.7% of cases would a significantly different decision on the interpretation and the need for supplementary testing have been made by considering the thyroxine result alone. These discrepancies could be readily avoided by using slightly wider limits for the performance of supplementary tests. It is concluded that the work and cost of the routine measurement of the free thyroxine index are not justified.