Finnish national screening for hypothyroidism. Few false positives, early therapy

National cord blood screening for congenital hypothyroidism has operated in Finland with complete coverage since 1980. A low frequency of false positives, 0.08%, was achieved by supplementing the TSH screen with a T4 determination in borderline samples. Among 175188 infants the incidence of (unconfirmed) hypothyroidism was 1/2637. The median age at start of therapy was 6 days. The programme imposed a 2-3 week therapy on the false positive cases. This did not appear to cause any adverse effects. A mechanism for masking congenital hypothyroidism was observed: two athyroid infants were euthyroid at birth because of feto-fetal transfusion.