Thyroxine (T4) radioimmunoassay using filter paper dried blood sample: an attempt for screening of neonates for hypothyroidism

This paper describes a sensitive but simple and less expensive method suitable for estimation of thyroxine (T4) level. Deficiency of iodine during fetal life results in neonatal hypothyroidism and critinism. Frequency of neonatal hypothyroidism is 1 in 5000 to 7000 in countries having iodine deficiency. It is therefore important to diagnose the neonatal hypothyroidism as soon as possible after birth. The estimation of thyroxine has been found to the a reliable index for diagnosis of hypothyroidism and has long been used for screening of neonatal hypothyroidism. In the present study, instead of serum sample, a 6 mm disc of filter paper containing dried blood sample was used. The test was carried out in the laboratory with 40 samples. As compared to the sensitivity of serum sample technique which is 15.19 n mol/L, the filter paper technique has the sensitivity of 17.23 n mol/L. The work revealed that the T4 concentration do not depend upon the amount of blood on the filter paper. Effect of temperature on filter paper disc was evaluated at 4o c, at 25o c and at 37o c. Results obtained showed significant variation and the best result was obtained for the sample kept at 4o c. The method is simple, rapid, less expensive and needs a small amount of blood and is, therefore, a useful technique for mass screening of neonatal hypothyroidism. 6 refs., 4 tables (author)