Radioimmunoassay of "free thyroxin" in dried blood spots on filter paper - preliminary observations on the effective differentiation of subjects with congenital hypothyroidism from those with subnormal thyroxin-binding globulin and normal subjects
Mizuta, H.; Miyai, K.; Ichihara, K.; Amino, N.; Harada, T.; Nose, O.; Tanizawa, O.
Clinical Chemistry 28(3): 505-508
1982
ISSN/ISBN: 0009-9147 PMID: 6802516 Document Number: 194221
In this sensitive, simple method for measuring free thyroxine (FT4) in eluates of dried blood spots on filter paper by use of a radioimmunoassay kit (Amerlex Free T4 RIA), the measurable range of FT4 is 1.8-57 ng/l (equivalent to the concentration in serum), or 7-237 fg/tube. The mean coefficients of variation for within assay-within spots, within assay-between spots, and between assays were 5.3, 5.0 and 6.2%, respectively. FT4 in blood spotted on filter paper is stable for at least 1 mo. when dried and kept at -20.degree. C, 4.degree. C, room temperature (.apprx. 25.degree. C) or 37.degree. C. The results for FT4 in dried blood spots correlated closely with the FT4 concentration in serum (r = 0.99). The method can be used to differentiate cases of primary and secondary hypothyroidism from normal subjects and those with subnormal thyroxine-binding globulin. This method may be useful in screening for congenital hypothyroidism, because sample-retesting is not necessary.