Determination of FT4 in whole blood on filter paper as screening examination for hypothyroidism

Using the reagents of the AMERLEX FT4 RIA Kit by AMERSHAM/BUCHLER, we developed a method for determining FT4 in whole blood on filter paper, which is suitable for automation and furnishes well reproducible results in no more than 90 minutes. During routine screening examinations for hypothyroidism TSH and FT4 were assayed in 6.500 newborns. In about 97% of all clinically normal newborns the call-back rate for FT4 assays is not higher than on TSH assaying (0.15%). The sensitivity and specificity of identifying hypothyroidism is apparently no worse than the diagnostic validity of TSH assays. FT4 assays offer additional diagnostic information on cerebral thyroid dysfunction and extrathyroid alterations of thyroid hormone metabolism. In diseased children at intensive care units repeat FT4 assays are suggestive prognostically. In newborns at a gestational age of less than 32 weeks and in diseased infants at intensive care units additional TSH assays are necessary if FT4 levels are reduced to differentiate between hypothyroxinemia and primary hypothyroidism. Provided our preliminary results are confirmed in larger series, FT4 assays can be recommended as a first step in the screening program for hypothyroidism. If FT4 levels are low, TSH can be determined in one and the same sample on the same day. As assays could be simplified and done more rapidly without any loss of diagnostic quality, considerable practical advantages could be gained. (Author)