Immunoassay of human TSH using dried blood samples.

A sensitive, semi-quantitative radioimmunoassay method to screen for elevations of TSH concentration in blood is described. The method requires two 0.32 cm dots of dried blood-impregnated filter paper (equivalent to 3 mul plasma) and a 3-day incubation. Separation of bound and free is obtained using polyethylene glycol. The method can recognize TSH concentrations as low as 22 muU/ml using a highly sensitive antiserum developed by one of us (AFP). TSH in dried cord and newborn blood from 4 infants with congenital hypothyroidism was clearly higher than in normal infants. The method is suitable for use in a newborn screening program to confirm the suspicion of primary hypothyroidism in specimens with low T4 concentrations.