AbstractThe recovery periods of serum thyroid stimulating hormone (TSH) were studied in patients with congenital hypothyroidism found on neonatal screening.L‐T4 was administered at 37.8 ± 15.1 days of age (M ± SD), and serum T4, T3, and TSH concentrations were measured. AfterL‐T4 therapy with 10 µg/kg, the serum TSH levels decreased to below 10 µU/ml in five patients within a week, and in another two patients, within two weeks, and in the total number of 12 patients out of 15 (80%), by one month.These results suggest that the recovery of serum TSH occurs rapidly in hypothyroid infants. Thus, the serum TSH concentration may be a good measurement for assessing the adequacy of thyroid hormone therapy from early infancy in patients with congenital hypothy