Serum thyroxine and thyroid stimulating hormone concentrations after treatment of congenital hypothyroidism.

Serum thyroid stimulating hormone and thyroxine concentrations were monitored in 42 infants who had been treated for congenital hypothyroidism. Serum thyroid stimulating hormone concentrations were raised in 22 of the infants (52%) at 2 to 4 months, in 16 (38%) at 5 to 11 months, in 14 (33%) at 12 to 18 months, and in eight (19%) at 2 to 4 years. Serum thyroxine and the dose of L-thyroxine/kg/body weight were significantly lower in those infants with raised thyroid stimulating hormone concentrations. Thyroid stimulating hormone was appropriately suppressed when the dose of L-thyroxine was increased, and only one child had delayed maturation of the hypothalamic/pituitary/thyroid axis. We believe it is the infant's rapid gain in weight in the first two years of life that necessitates this decrease in the dose of L-thyroxine/kg body weight and recommend that the treatment of this age group is reviewed every two to three months.