[Value of the ultrasensitive assay of plasma TSH for the surveillance of replacement treatment in children with congenital hypothyroidism].

This study proposes some improvements in monitoring congenital hypothyroidism (CH) using an ultrasensitive plasma TSH assay (usTSH). Patients were 42 CH treated with levo-thyroxine (L-T4) for at least 6 months. Controls were 25 age-matched children (C). Comparative determinations of plasma TSH with classical radioimmunoassay (RIA), on one hand, and usTSH on the other hand, revealed RIA values to be beneath the sensitivity threshold in 3 C and in 15 CH. With usTSH, all the TSH values were assayed over the sensitivity threshold. Therefore the CH could be placed into 3 subgroups according to whether their values were below (n = 17), equal to (n = 15) or above (n = 10) those determined for C. Mean plasma levels of thyroxine (T4) or free thyroxine (FT4) were higher in the CH group, considered as a whole, than in C (p less than .01). Furthermore these values did not appear to be correlated with those in the TSH subgroups, anymore than they were with those of therapeutic doses of L-T4 administered. These discrepancies may be explained in terms of metabolism of exogenous thyroid hormones. UsTSH plasma values would therefore reflect the action of thyroid hormones on thyreotropic cells. To this extent the assay constitutes a sensitive index in detecting both therapeutic insufficiencies and overtreatments.