Becent studies have suggested that maternal TSH receptor-blocking antibodies might be of primary etiological importance in some cases of transient congenital hypothyroidism (CH).Because these antibodies are extremely potent, we evaluated the feasibility of identifying babies at risk by using readily available newborn blood spots.Blood spots obtained from 84 normal babies (group 1) and from 354 infants whose initial T, was less than the tenth percentile for the assay and whose TSH was 40 mU/L or more (group 2) were studied without knowledge of the diagnosis.Blood was eluted from spots overnight and evaluated for ['161]TSH binding inhibition (TBI) to solubilized porcine thyroid membranes.Four spots obtained from 3 group 2 babies, but none of those from the group 1 infants, exhibited TBI activity greater than 3