Outcome of lower L-thyroxine dose for treatment of congenital hypothyroidism

The appropriateness of the recommended L-thyroxine dose (10-15 micrograms/kg/day) for the treatment of congenital hypothyroidism has been questioned because of the risk of iatrogenic hyperthyroidism. We report the outcome of 23 newborns with congenital primary hypothyroidism treated with 25 micrograms L-thyroxine per day (5.3-9.2 micrograms/kg/day) and followed for an average of 59 months. Serum thyroxine (T4) values increased (X = 11.4 +/- 2.7 micrograms/dL) within 4 weeks posttherapy; eight infants had T4 levels > or = 13 micrograms/dL on only half the currently recommended dose. Thyroid-stimulating hormone (TSH) values remained elevated in 18 of 21 patients for 2-21 months despite a high-normal T4. Psychometric tests were performed in 19 of the 23 patients. The mean Full Scale IQ for the congenital hypothyroid group (n = 16) was 101.4 +/- 13.2 with comparable Verbal and Performance IQ scores. Patients with a bone age (BA) of < or = 32 weeks or T4 < 2 micrograms/dL at initial evaluation had significantly Lower Verbal IQ scores. A standardized parent-report assessment of behavioral and emotional functioning revealed subgroup scale scores that were indistinguishable from nonclinical norms. We conclude that (1) average range IQ scores and positive behavioral adaptation are observed in congenitally hypothyroid children treated with L-thyroxine doses lower than currently recommended; (2) the L-thyroxine dose should be individualized to prevent iatrogenic hyperthyroidism; (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.