Thyroid function and 3,3'-diiodothyronine sulfate cross-reactive substance (compound W) in maternal hyperthyroidism with antithyroid treatment

To test whether the serial measurement of maternal levels of compound W, a 3,3'-diiodothyronine sulfate cross-reactive substance, can serve as a potential indicator of fetal thyroid function in pregnant women receiving antithyroid medication. Compound W was measured repeatedly in serum of pregnant women with hyperthyroidism treated with antithyroid medication. Free thyroxine levels of mothers and serum thyroid-stimulating hormone levels of 1-day-old neonates were analyzed by local clinical or state laboratories. Use of minimal antithyroid medication impaired the progressive increase of compound W seen in euthyroid mothers during pregnancy. At term, depressed compound W levels in maternal serum were found in 7 of 22 pregnancies; in 1 case, maternal compound W was suppressed and newborn thyroid-stimulating hormone was elevated. Seven mothers with treated hyperthyroidism failed to show an increase in serum levels of compound W after midterm. Normal progression of maternal serum compound W may be an index of normal fetal thyroid development in mothers with hyperthyroidism treated with necessary antithyroid medication.