Abstract
The usefulness of Thyopac-4 test, a new competitive protein binding analysis for the determination of total thyroxine concentration, was studied fundamentally and clinically. The test vial contains specific adsorbing granules which do not bind T4irreversibly, and incubation time for 30 min was found long enough to reach complete equilibrium. The effect of incubation temperature was also not so critical, and only a little effect was observed in high T4range exceeding 20μg/100ml. Variability among kits by using pooled serum was 1.61%, and reproducibility by duplicatedly measured 11 patients' sera was excellent and the maximal difference among them was 0.9μg/100 ml. The reciprocal plotting of radioactivity in the incubation supernatant lead the stand-ard curve linear. The linearity of the standard curve was confirmed by standard T4serum samples with 0, 3.6, 7.3, 10.9, 14.6 and 18.2μg/100 ml T4.
Further, utilization of standard serum instead of standard T4made the correction of extraction rate unnecessary.
Clinically the test was appraised for 80 cases with various conditions, using same serum samples, T4by Tetrasorb and thyroxine binding capacity by Thyopac-3 and Triosorb were also determined. The test results were as follow: normal subjects (35 cases) 8.8±2.2μg/100 ml, hyperthyroidism (20 cases) 18.6±2.7μg/100 ml, hypothyroidism (19 cases) 3.9±1.9μg/100 ml and pregnancy 10.0±1.3μg/100 ml, and good differentiation was observed between hyperthyroidism and normal subjects but slight overlapping was found between normal subjects and hypothyroidism. Comparing these with T4by Tetrasorb, a good correlation (r=+0.89) was observed, but Thyopac-4 values had slight tendency of showing higher values in low T4range and lower values in high T4range.
As an index for serum free T4, free Thyopac index (Thyopac-4/Thyopac-3) was calculated. The index improved the observed high T4in pregnancy, and was consid-ered as a better parameter, however, this index could not differentiate normal subjects from hypothyroidism completely. Another free T4index was calculated according to our perviously reported formula (T3RUXT4/1-0.6 T3RU) . Using this index, normal subjects were clearly differentiated from hypothyroidism.
With many technical advantages and good clinical results, Thyopac-4 test was considered to be a very useful routine test. Free Thyopac index as an index for free T4was also found reasonable, however, consideration for free TBPA capacity may achieve further improvement.
