Falsely elevated serum thyrotropin (TSH) in newborn infants: transfer from mothers to infants of a factor interfering in the TSH radioimmunoassay.

In a TSH screening program for congenital hypothyroidism we detected seven newborn infants with normal plasma T4 and T3 levels but high immunoassayable TSH. Similar findings were obtained in their mothers. Serial plasma dilution curves, with and without the addition of normal rabbit serum to the samples, showed that the result of TSH assay performed with antihuman TSH rabbit antiserum was falsely elevated in mothers and infants by an interfering factor. Follow-up of the infants demonstrated that the falsely elevated plasma TSH levels returned to normal within the first 6 months of life. On the contrary, plasma TSH levels remained high in the mothers. These results suggested a placental transfer of maternal antibodies. Indeed, the analysis of the mothers anamnesis revealed that all had previously received injections of a microbial vaccine cultured on a rabbit lung-containing medium. We conclude that placental transfer of a maternal antirabbit factor may cause an artefactual hyperthyrotropinemia in the newborn and the incorrect diagnosis of neonatal hypothyroidism. This can be avoided by the addition of normal rabbit serum or immunoglobulin to the TSH RIA tubes.