Elimination Kinetics of Maternally Derived Thyrotropin Receptor–Blocking Antibodies in a Newborn With Significant Thyrotropin Elevation

• Objective. —To determine the course of maternally derived elevations in thyrotropin-binding inhibitory immunoglobulins in a neonate. Design. —Case report. Setting. —University pediatric endocrinology clinic and endocrine immunology laboratory in Ohio. Participants. —An infant with elevated thyrotropin levels but near-normal total thyroxine levels, and her mother. Interventions. —None. Measurements/Main Results. —Thyroid hormone, thyrotropin, and thyrotropin-blocking immunoglobulin concentrations were serially measured in a woman and her infant, who was found to have elevated thyrotropin levels (234 mU/L) and borderline low thyroxine levels (95 nmol/L). As infant thyroxine concentrations remained normal (125 to 145 nmol/L), no thyroxine supplementation was given. Thyrotropin levels decreased concomitantly with thyrotropin-blocking inhibitory immunoglobulin levels, and normalized by day 56 of life. The apparent elimination half-life of thyrotropin-blocking immunoglobulins was 7.5 days. Conclusions. —The observed parallel elimination kinetics suggest that the thyrotropin receptor antibody acts as a thyrotropin antagonist, resulting in compensatory thyrotropin elevations. The duration of such elevations may be predicted on the basis of such elimination. ( AJDC. 1992;146:1074-1077)