OUTCOME OF LOWER L-THYROXINE DOSE FOR TREATMENT OF CONGENITAL HYPOTHYROIDISM

The appropriateness of the recommended L-thyroxine dose (10-15 mu g/kg /day) for the treatment of congenital hypothyroidism has been question ed because of the risk of iatrogenic hyperthyroidism. We report the ou tcome of 23 newborns with congenital primary hypothyroidism treated wi th 25 mu g L-thyroxine per day (5.3-9.2 mu g/kg/day) and followed for an average of 59 months. Serum thyroxine (T-4) values increased (X=11. 4+/-2.7 mu g/dL) within 4 weeks posttherapy; eight infants had T-4 lev els greater than or equal to 13 mu g/dL on only half the currently rec ommended dose. Thyroid-stimulating hormone (TSH) values remained eleva ted in 18 of 21 patients for 2-21 months despite a high-normal T-4. Ps ychometric tests were performed in 19 of the 23 patients. The mean Ful l Scale IQ for the congenital hypothyroid group (n=16) was 101.4+/-13. 2 with comparable Verbal and Performance IQ scores. Patients with a bo ne age (BA) of less than or equal to 32 weeks or T-4<2 mu g/dL at init ial evaluation had significantly Lower Verbal IQ scores. A standardize d parent-report assessment of behavioral and emotional functioning rev ealed subgroup scale scores that were indistinguishable from nonclinic al norms. We conclude that (1) average range IQ scores and positive be havioral adaptation are observed in congenitally hypothyroid children treated with L-thyroxine doses lower than currently recommended; (2) t he L-thyroxine dose should be individualized to prevent iatrogenic hyp erthyroidism; (3) TSH normalization should not be a primary objective of treatment, and (4) a prospective study comparing the advantages and risks of different doses of L-thyroxine is needed.