Transient neonatal hyperthyrotropinemia: a factitious syndrome due to the presence of heterophilic antibodies in the plasma of infants and their mothers.

In a national screening program for neonatal hypothyroidism, our criterion for diagnosis is a plasma TSH value exceeding 40 microunits/ml on the fifth day of life. Excluding cases with obvious thyroid abnormalities, we identified a group of 14 neonates with unexplained plasma TSH elevation out of 10,261 screened. The mothers of these infants also exhibited high plasma TSH immunoreactivity; both mother and child in these instances were clinically euthyroid. The material responsible for this TSH immunoreactivity had the following characteristics. After parturition, it disappears from circulation within 2 months in the infants and within 4-6 months in the mother, its apparent molecular weight approximated 150,000 daltons, and its affinity for protein A-Sepharose and its binding properties suggested that the material was an immunoglobulin. Binding is observed typically for heterologous immunoglobulin and human TSH. The heterophilic character of these immunoglobulins explains their interference in many RIA systems, including the TSH RIA commonly used in neonatal screening programs for hypothyroidism.