The method of T4 radioimmunoassay using filter paper blood samples and its application for the early detection of neonatal hypothyroidism

Abstract

The method of radioimmunoassay of the Disc T4 (dried blood spot T4) was examined for the purpose of mass screening for neonatal hypothyroidism and compared with a parallel assay of T4 and TSH in dried blood spotted on filter paper at the fifth day of life. By changing the volumes of T4 antibody and T4 antigen solutions, it is possible to measure thyroxine using two discs 3 mm in diameter. Disc T4 values obtained in 45 unknown samples by this method were in good correlation with serum T4 values of the same subjects. The coefficients of variation were 5.6 to 18.9% in the range of 5.3 to 10.7 μg/100 ml of Disc T4 (within assay), and 7.9 to 20.8% in the range of 2.4 to 10.3 μg/100 ml of Disc T4 (between assay). Both Disc T4 and Disc TSH of 26,067 newborn infants born between June 1978 and May 1979 were measured. Out of 26,067 measurements, four cases of primary hypothyroidism, fourteen cases of TBG deficiency and two cases of transient hypothyroidism were discovered. Out of the four cases of primary hypothyroidism, three showed low Disc T4 (1.3 to 4.0 μg/100 ml) and high Disc TSH (200 to 240 μu/ml), one showed normal Disc T4 (7.1 μg/100 ml) and high Disc TSH (10 μu/ml). It was thought the last case might be mild hypothyroidism. In 14 cases of TBG deficiency, Disc T4 was not detectable to 3.4 μg/100 ml while Disc TSH was within the normal range. In two cases of transient hypothyroidism, Disc T4 measurements showed normal range (6.5, 10.6 μg/100 ml), and Disc TSH showed high values (41, 120 μu/ml). Therefore, Disc TSH was more sensitive and showed less false positive cases than did Disc T4 for screening hypothyroidism. Although no secondary hypothyroidism existed in the 26,067 measurements, it continues to be important to measure Disc T4 for the discovery of secondary or tertiary hypothyroidism.