An improved assay of thyrotropin in dried blood samples on filter paper as a screening test for neonatal hypothyroidism.

A method for measuring thyrotropin (TSH) in eluates of dried blood samples on filter paper was evaluated and improved as a screening test for neonatal hypothyroidism. A linear relationship between the volume of eluate and the TSH value and good recoveries of endogenous TSH (104%) and added TSH (89%) were obtained, indicating that TSH in dried blood was extracted well by overnight elution and determined accurately by radioimmunoassay. The TSH content in the central portion of a spot was less (71%) than that in the peripheral portion. The TSH in dried blood samples on filter paper was stable at 4 degrees C, room temperature (about 25 degrees C) or 37 degrees C for 1 month. The TSH values of eluates were correlated with those of whole blood (r = 0.90) and serum (r = 0.81). Cases of primary hypothyroidism cound be readily differentiated from normal subjects by this method, even when eluates of their blood were combined with those of normal blood for assay of TSH.