Symptoms and signs of severe hypothyroidism developed in a young woman at age 15. These symptoms progressed for a year; at age 16, she was found to have a firm goiter, thyroid autoantibodies, very low serum thyroxine and high thyrotropin values, indicating autoimmune thyroiditis with hypothyroidism. She received L-thyroxine, 0.20 mg per day, and was well until age 24 when she became pregnant. In the first trimester, manifestations indicative of hyperthyroidism developed; these were only ultimately recognized immediately after delivery of a 32-week still-born goitrous baby. Despite the discontinuation of thyroxine therapy, the hyperthyroidism persisted and was confirmed as Graves' disease by elevated thyroxine, triiodothyronine, and radioactive iodine uptake values, a diffuse scanning result, and the presence of thyroid-stimulating antibody. The patient was treated with propylthiouracil and became pregnant while receiving that regimen. Later, several months after delivery, the patient was treated with radioactive iodine, ultimately became hypothyroid, and has been treated ever since with thyroxine. She became pregnant again and, because of the continuing high titers of thyroid-stimulating antibody, received propylthiouracil, 100 mg daily, commencing in the third trimester of pregnancy, to avoid probable fetal hyperthyroidism due to the transplacental transfer of thyroid-stimulating antibody. In each of the last two pregnancies, when the infants were born, they seemed normal (because of the transplacental effect of propylthiouracil), but passive-transfer neonatal hyperthyroidism developed in each within 10 days after delivery, ultimately requiring treatment by conventional means. This case illustrates the following points: (1) Hyperthyroidism occasionally develops years after hypothyroidism. (2) In young women, high titers of thyroid-stimulating antibody may produce fetal and neonatal passive-transfer hyperthyroidism even at a time when the mother herself is no longer hyperthyroid; transplacental treatment of the fetus by maternal propylthiouracil ingestion may thus be necessary during the last trimester, but only when there is a high degree of probability that the fetus is at risk. (3) Because the infants had been protected in utero by the placental transfer of propylthiouracil, neonatal hyperthyroidism did not develop until several days after delivery.(ABSTRACT TRUNCATED AT 400 WORDS)