Transplacental passage of thyrotropin (TSH)-binding inhibitory immunoglobulins may result in transient congenital hypothyroidism. We measured serum TSH-binding inhibitory index (TBII) in 11 infants with abnormal screening findings using a commercially available kit. Two of the infants, who were siblings, had markedly elevated TBII values (90% and 100%, respectively), as did their mother (89%, 100%), and had a clinical course consistent with transient antibody-mediated hypothyroidism. Four other infants had a borderline or mildly elevated TBII that was not present in maternal serum, suggesting that endogenous TSH was being measured in this assay. The TBII was measured in the sera of 18 additional children with primary hypothyroidism and in human TSH standards from 25 to 2000 mU/L. Increasing concentrations of TSH were associated with a linear increase in TBII. Measurement of TBII by this method may identify infants with transient antibody-mediated hypothyroidism, although simultaneous assessment of maternal serum is necessary.