ESPE Abstracts

Background: Treatment of congenital primary hypothyroidism (CPHT) fluctuates between two opposite risks for the neuro-cognitive development in a critical window during the neonatal period, under- and over-treatment.

Objective: To investigate the relevance of current international recommendations of 10–15 μg/kg per day for L-thyroxine (lT₄) doses in CPHT.

Method: fT₄ and TSH were measured in a plasma sample after 15 days and one month of lT₄ treatment in 90 healthy full term newborns diagnosed with congenital hypothyroidism from routine screening on the 3rd day of life over a period of 9 years 3 months who were treated with doses of a liquid preparation of lT₄, ranging from 6.3 (2.5th percentile) to 11.6 (97.5th percentile) (median: 8.0) μg/kg per day. fT₄ and TSH levels at the 15th day of treatment served to adjust lT₄ doses.

Results: At the 15th day of treatment, only 4.8% of the newborns had a plasma fT₄ level below the lower value of 12 pmoles/l, while 46.7% and 32.2% had a plasma fT₄ level above the upper values of 25 or 30 pmoles/l respectively. A significant independent positive correlation (r²=0.2, P=0.0027) was observed between fT₄ plasma levels at the 15th day of life and the initial lT₄ dose. The relationship between 15 day lT₄ dose and fT₄ at 1 month of treatment was weaker.

Conclusions: Although significant enough, the correlation between initial lT₄ doses and fT₄ level at the 15th day is characterised by a dispersion inducing a frequent overtreatment by liquid lT₄ administration, when following recommended 10–15 μg/kg per day dosages in newborns with CPHT, whose neuro-cognitive consequences are to evaluate.

Funding: French National Insurance Organization (Fund for Neonatal Screening).