Serum thyrotrophin determination on day 5 of life as screening procedure for congenital hypothyroidism.

In 327 newborns cord blood thyroxine (T4) was 11.8 +/- 0.4 mug/100 ml (SEM) (151.9 +/- 5.1 nmol/l), and serum thyrotrophin (TSH) 6.7+/-1.0 muU/ml. Variability was marked for both T4 and TSH. Remeasured in the same patients on the fifth day of life, the TSH level was 3.7 +/- 1.0 muU/ml, lower than at birth (P less than 0.001), while scattering of TSH values was much smaller, with 99.4 % of values less than 12 muU/ml..