SummaryOBJECTIVEThe aim of this study was to develop an assay for the measurement of thyroid blocking antibodies (TBAb), based on the ability of patient serum to inhibit TSH stimulated3H‐cAMP production following incubation of FRTL‐5 or JPOS cells with3H‐adenine. The assay was then used to evaluate a child born with neonatal hypothyroidism.DESIGNThe levels of TBAb, TSAb (thyroid stimulating antibodies), TBII (TSH binding Inhibitory antibodies), and the thyroid antibodies anti‐thyroid peroxidase and thymoglobulin antibodies were measured in both mother and child over a 6‐month post‐natal period.PATIENTThe assay for TBAb was used to evaluate a child born with neonatal hypothyroidism whose mother had a history of hypothyroidism due to Haehimoto's thyrolditis. A99mTc pertechnetate scan showed no evidence of functioning thyroid tissue. At 20 months of age an ultrasound verified a normally positioned thyroid.RESULTSInitially, high levels of TBII and antithyroid antibodies were present in the serum of both mother and child. In both, the levels of TSAb were undetectable but there were significant levels of TBAb. The levels of TBAb decreased to control levels in the child within 2 months of birth but remained elevated in the mother's serum.CONCLUSIONSThis case of neonatal hypothyroidism associated with the passage of thyroid blocking antibodies demonstrates the utility of this new assay in the differential diagnosis of neonatal hyp