RESUMEN
BACKGROUND: There have been reports of patients with primary hypothyroidism not identified by TSH measurement due to a presumably delayed rise in serum TSH. However, there are no data on the incidence of false negative results in neonatal screening programs employing primary TSH assay for diagnosis. AIM: To investigate the incidence of false negative results in a neonatal screening program using the primary TSH approach and evaluate a strategy to avoid misdiagnosis. INFANTS AND METHODS: 190 newborns, with initial TSH > 15.0 IU/l and < 20.0 IU/l (screening cutoff). These infants were submitted to a second TSH measurement around 30 days after the first screening. RESULTS: Thirty days after the first screening, four of the 190 infants (2.1%) remained with TSH levels around the cut-off screening level or higher. Of these four patients, three had an absence of thyroid gland on ultrasonography, with a final diagnosis of dysgenesis. The fourth patient had a normal shaped gland in its usual location by ultrasonography, with an outcome of transitory congenital hypothyroidism. CONCLUSIONS: The incidence of missed congenital hypothyroidism diagnoses in this neonatal screening program based on the TSH approach was low and acceptable. Nevertheless, with the proposed strategy, the risk of false negative results can be reduced without significant impact on the overall cost of the screening program.