Optimum replacement dose of thyroid hormone assessed by highly sensitive TSH determination in patients with congenital hypothyroidism.

Serum thyroid hormone concentrations were measured in 100 samples from 25 patients with congenital hypothyroidism who were clinically well while receiving L-T4 therapy. Thyroxine concentrations were significantly higher than those of controls (p less than 0.01), while triiodothyronine was not significantly different. These samples were divided into four groups according to serum thyroid stimulating hormone concentrations as measured by highly sensitive immunoradiometric assay (IRMA-TSH). Serum thyroid hormone concentrations were compared among groups. The replacement dose of L-T4 and serum thyroid hormone in groups with undetectable IRMA-TSH were significantly higher than those in groups with normal or increased IRMA-TSH. These results show that serum thyroxine concentrations increase in most patients with congenital hypothyroidism on L-T4 therapy. Therefore, thyroxine concentrations above normal are not necessarily of clinical significance if IRMA-TSH is detectable. Undetectable IRMA-TSH might indicate the necessity for a reduction in the L-T4 replacement dose in patients with congenital hypothyroidism.