Treatment of maternal hyperthyroidism with antithyroid agents and changes in thyrotrophin and thyroxine in the newborn.

Eleven pregnant women with concomitant hyperthyroidism were treated with antithyroid drugs. At monthly intervals serum thyroxine (T4) and triiodothyronine (T3) were measured with radioimmunoassay, the Sephadex uptake of radioactive triiodothyronine (T3U) determined the free T4 and T3 indices calculated (FT4I, FT3I). TSH-binding inhibiting immunoglobulins (TBII) were determined by the radiomembrane assay. Serum TSH and T4 were measured at delivery from cord blood and/or from the newborn infants some days after birth. Serum TSH was significantly elevated in one infant. There was an inadequate post-partal rise in serum T4 concentration in this child and in another who showed only a marginal elevation of TSH. The mothers of these infants were given carbimazole in doses of 30 and 25 mg/day, respectively, at the time of delivery. No significant changes were seen in other infants, the daily doses being 20 mg of carbimazole or less. There was no clinical indication of hypo- or hyperthyroidism in any of the newborn. The TBII were positive in most patients and there was a trend of normalization during treatment. No relationship between the dose of antithyroid drug and the level of TBII could be seen. During treatment the dose was adjusted according to the FT3I values. This seems to be an adequate laboratory test for this purpose.