Anomalous thyrotrophin in a newborn infant due to anti-thyrotrophin antibody

A baby was found to have an elevated thyrotrophin (TSH) value on neonatal screening but was clinically euthyroid. Repeat TSH measurements confirmed the elevated levels. Investigation of the mother revealed a similar finding with elevated TSH, normal free thyroxin value and euthyroid clinical state. TSH values using the Farmos TSH RIA kit were 3.5 MU/L for the mother and 7.8 MU/L for her son. This suggested an interference in the previous TSH assays. Measurement of anti-thyrotrophin receptor antibody also produced anomalous results of -57 U/L in the mother and -15 U/L for the son (reference range -13 to 13 U/L). The presence of an anti-TSH antibody was determined by incubating serum with I-TSH followed by immunoglobulin separation with protein-A sepharose. After centrifugation and washing the immunoglobulin bound I-TSH was counted in a gamma counter. This showed binding of 39.5% of tracer in the mother compared with less than 2% In controls. The interfering antibody was acquired by the son transplacentally. On follow up the interference has remained in the mother’s serum but has decreased in her son’s. TSH antibodies causing anomalous TSH values are rare and the reason for their development in the mother unknown.