Screening for minimal hypothyroidism. How sensitive is the free thyroxine assay?

The free thyroxine index (FTI) and a free thyroxine (FT4) assay were compared with the thyrotropin-releasing hormone stimulation test in 98 consecutive patients to determine whether the FT4 assay is more sensitive than the FTI in screening patients for minimal hypothyroidism. FTI and FT4 values did not agree with the clinical evaluation in 20% (3/15) of the patients with a transitional thyroid-stimulating hormone (TSH) response (21 to 35 microU/ml) and in 64.3% (9/14) of the patients with an augmented TSH response (greater than 35 microU/ml). From these results we conclude that the FT4 assay is no more sensitive than the FTI in detection of such minimal thyroid dysfunction.