Elevated free thyroxine levels detected by a neonatal screening system

In Sapporo city of Japan, neonatal screening for congenital hypothyroidism has used the measurement of free thyroxine (T4) and thyroid-stimulating hormone (TSH) in the filter-paper blood spot. This system has enabled us to identify hyperthyroxinemic diseases. Filter papers were collected from neonatal infants born at 4-6 d of age and neonates who showed elevated free T4 (>4.0 ng/dL, 4 SD above the mean) were studied. Between January 2000 and December 2006, 83,232 newborns were screened. Eleven infants demonstrated persistent hyperthyroxinemia. One patient with slightly elevated free T4 and normal TSH was diagnosed as having familial dysalbuminemic hyperthyroxinemia (FDH). The other two patients with elevated free T4 without suppressed TSH were considered as having resistance of thyroid hormone (RTH), and analysis of thyroid hormone receptor (TR) beta gene confirmed the diagnosis. The remaining eight patients were diagnosed as having neonatal Graves' disease (NGD). Seven of eight pregnant women were treated with antithyroid drug and thus only one unrecognized NGD during pregnancy was detected by screening. Our screening system enables for early awareness of RTH and FDH. Regarding Graves' disease, the benefit of elevated free T4 screening is small, because most pregnant women with Graves' disease were managed.