The importance of thyroglobulin levels in monitoring the treatment of congenital hypothyroidism.

We have previously reported on high thyroid-stimulating hormone (TSH) concentrations in clinically euthyroid children with congenital hypothyroidism (CH) undergoing appropriate treatment. Whether this TSH is biologically active or not is still unclear. It has been shown that ectopic thyroid tissue does not involute during thyroxine (T4) therapy and thus can continue to secrete thyroglobulin (Tg). This study was undertaken to determine whether the Tg levels in ectopic CH infants represent residual thyroid tissue stimulated by biologically active TSH and whether this Tg can be used to help monitor CH treatment. Among the 51 primary CH children (age 2-14 years) diagnosed and followed up by us, 28 had measurable Tg values (> 2 pmol/l) several years after the T4 treatment had been started. In 8 of the children, Tg was measured as early as the time of diagnosis and followed up for at least 3 years. The Tg levels decreased much more slowly than the TSH levels did, and secondary Tg rises were observed. By 5 months of age, all children had Tg levels less than 25 pmol/l. Although in some infants the Tg levels paralleled TSH behavior, in others the TSH-Tg correlation was not so obvious. In another group of 8 children who had high TSH values despite normal T4, the LT4 replacement dosage was increased by 60% for 1 week (from 3.5 +/- 0.2 to 5.5 +/- 0.5 micrograms/kg/day) in order to examine the TSH-Tg dependence.(ABSTRACT TRUNCATED AT 250 WORDS)