The adequate L-thyroxine dosage for the initial treatment of infants with congenital hypothyroidism is a subject of controversy. Some recommend higher dosages (> 10 micrograms/kg/day) to ensure adequate levels, while others advocate lower dosages to permit normalisation of thyroid status. The aim of this study was to evaluate the results of a treatment strategy using an initial dosage of 7.5-8.0 micrograms/kg per day, TSH measurements being taken at 15 and 30 days of treatment. Fifty one newborns infants with primary congenital hypothyroidism detected by neonatal screening were treated with the same therapeutic strategy. A mean L-thyroxine dosage of 7.9 micrograms/kg per day at the onset of treatment and 6.6 micrograms/kg/d at 2 months, normalised the FT4 and FT3 levels at 15 days in 100% and TSH levels at 2 months in 90% of cases. Many patients showed elevated levels of FT4 and a systematic higher initial dosage could expose many infants to a dangerous hyperthyroidism. Patients with abnormal TSH levels at 2 months already had higher TSH levels in the first 8 weeks of life and, despite higher L-thyroxine dosage, also exhibited lower FT4 and FT3 levels. These patients who needed an early increase in dosage had already shown a more profound ante and neonatal hypothyroidism. This subgroup of patients require a higher dosage of thyroxine and early assessment of FT4, FT3 and TSH levels are required for optimum dosage choice.
Conclusion: Even though a subgroup of patients may require a higher dosage of L-thyroxine, an initial dosage of 7.5-8.0 micrograms/kg per day, with an early assessment of FT4, FT3, and TSH levels, is adequate for the treatment of the majority of infants with congenital hypothyroidism.