Congenital hypothyroidism. Therapeutic strategy at the early stage of treatment

: The aim of this work was to determine the optimum dosage of L-thyroxine (L-T4) given to infants with congenital hypothyroidism (CH). Thirty seven hypothyroid infants diagnosed through the French screening programme for CH have been treated in our clinic. The study analysed the biochemical parameters (TSH, FT4, FT3) and the L-T4 doses during the first year of life. Treatment was started at 23 days of age (range: 13 to 37). A dose of 7.5 micrograms/kg/d of L-T4 was given at diagnosis. After 2 weeks of treatment, FT3 was normal and FT4 at the upper limit of the normal values. At that time, TSH plasma levels were normal (less than 6 micro UI/ml) in 47% of cases. After 1.7 month of treatment, 22% of patients had TSH levels greater than 10 micro UI/ml despite normal FT4 and FT3. This group of patients, despite being given an identical L-T4 dose, had a significantly lower FT4. They were not different from those who normalized TSH levels in terms of etiology, delayed bone maturation and levels of FT4 or FT3 at diagnosis. In conclusion, an initial dose of 7.5 micrograms/kg of L-T4 normalized FT4, FT3 and TSH in 80% of our patients. Twenty percent of patients seem to need more L-T4 to bring TSH levels back to normal at the end of the second month of treatment.