| Abstract: | Using the reagents of a commercially available test kit (Henning, Berlin) for the determination of free thyroxin (FT4) in serum we developed a radio immuno assay procedure to measure FT4 in whole dried blood on filter paper. The inter assay coefficient of variation was 19.1%, 10.7%, and 11.0% for the hypo-, normo-or hyperthyroid range for whole blood on filter paper. The corresponding within assay values were 7.0, 10.3, and 5.3% respectively. In 15,793 samples of dried blood on filter paper which were collected on the 5th or 6th day of life for the screening of inborn errors of metabolism or hypothyroidism FT4 was measured in addition to our routine determination of thyroid stimulating hormone (TSH). The mean concentration of FT4 was 20 to 22 pg/ml. 97,8% of all results were found between 10 and 35 pg/ml. Connatal hypothyroidism was confirmed clinically in 5 children showing FT4-values of 3.5, less than 2.0, less than 2.0, 3.4, and 6.2 pg/ml. The corresponding TSH-values were greater than 224, greater than 224, 99, 203, and 129 microU/ml. To make sure that a sufficient amount of blood had been eluted from the filter paper we measured the concentration of hemoglobin as an additional parameter. Low FT4 together with low hemoglobin concentrations were obtained in 18 samples indicating that a mismeasurement of FT4 might have occurred. We suggest that the determination of TSH in doublicates, which is the routine screening procedure in West Germany might be replaced by a combined determination of FT4 and TSH. |
