Aim: To assess thyrotropin (thyroid-stimulating hormone; TSH) suppression and serum thyroxine (T<sub>4</sub>) concentrations in infants with congenital hypothyroidism in relation to T<sub>4</sub> dose and pretreatment parameters. Method: A retrospective study of all cases treated in a single centre since neonatal screening began was performed. Results: In 54 infants treated with a mean daily T<sub>4</sub> dose of 9.8 µg/kg, the TSH concentration was suppressed (<6 mU/l) in 65% of the cases by 6 months with the serum T<sub>4</sub> level at the upper end of the infant reference range. Infants who suppressed their TSH later did not differ in pretreatment serum TSH or T<sub>4</sub> concentration. T<sub>4</sub> dose and serum T<sub>4</sub> level were lower in infants whose TSH was not suppressed. Conclusions: TSH suppression in congenital hypothyroidism is significantly related to serum levels and dosage of T<sub>4</sub>. We suggest that a delay in TSH suppression is mainly due to undertreatment.