In view of the increasing number of in vitro tests of thyroid function standardization of the biochemical assessment of patients with suspected thyroid disease was attempted. In 145 patients, measurements were made of serum total triiodothyronine (T3), tyroxine (T4) and thyrotropin (TSH) by the highly sensitive methods using the Delfia system. For comparative purposes, Delfia results were compared with those obtained by three different immunoassays: RIA, LIA and FPIA. The correlation between all tested methods over a wide range of concentrations was good, indicating very close agreement in the ranking of results. Correlation coefficients in all cases were higher than 0.90 and the intercepts did not differ significantly from zero. On the basis of the obtained results, the sensitivity, specificity and diagnostic power of the total T3, total T4 and TSH measurements for discrimination between euthyroid, hypothyroid and hyperthyroid patients was determined. TSH assay showed the highest sensitivity and diagnostic power as compared to total T3 and T4 assays. A significant, negative correlation was found in all studied patients population between TSH and T3 and TSH and T4 (r = -0.564 and r -0.695 respectively). These results seems to suggest that: 1) a detectable, normal TSH level indicates that the patient is euthyroid, and 2) a raised or lowered TSH level should be followed by measurement of T4 rather than T3 to distinguish between euthyroism and hypothyroidism and hyperthyroidism. This study also suggests that for practical purposes the best single screening test of thyroid function is TSH measured with a sensitive immunometric assay.

Keywords: Thyroid function, in vitro thyroid testing, immunoassays