Free T3 levels to monitor 1T4 therapy in congenital hypothyroid (CH) children during the first two years of life

Abstract

An elevated threshold for TSH suppression has been previously described in children during the first months of replacement therapy. We studied 15 children, diagnosed by neonatal CH screening during the first two years of life. All of them were clinically euthyroid in our multidisciplinary follow-up program(normal growth rate, absence of hypo/hyperthyroidism symptoms, normal neuropsychological development). Clinical evaluation and hormonal profiles (fT3,fT4,T3,T4,TSH) were assessed every three months. We found a significant correlation between total hormones, fT4,lT4 dosage and TSH levels. TSH levels were well above the normal range during the first year of life,varying widely between and within patients;fT3 concentrations were not correlated with TSH levels and were well in the normal range during the study. The correlation between lT4-TSH, fT4-TSH and lT4-fT4 shows that increasing the dose of lT4 decreases TSH levels according to an elevated threshold for TSH inhibition by an increase in fT4 levels but TSH does not fall in the normal range at the recommended therapeutic dose. This increase in fT4 and decrease in TSH does not seem to affect the thyroid status, since fT3 does not change for individual compensatory mechanism in fT4/fT3 conversion in the usual therapeutic range; increase of lT4 to completely inhibit TSH probably renders patients hyperthyroid.FT3 seems to be the best index for therapy.