A 16-year-old pregnant Puerto Rican woman who had been treated for thyrotoxicosis previously was evaluated for goiter, increased total thyroxine (T4) and triiodothyronine (T3) and free T4 estimate, despite a normal thyroid-stimulating hormone (TSH) concentration. These findings are consistent with a TSH-producing pituitary adenoma or the syndrome of generalized thyroid hormone resistance. However, sera from the patient, her mother and subsequently her newborn daughter demonstrated the increased albumin binding of T4 but not T3 that is characteristic of familial dysalbuminemic hyperthyroxinemia (FDH). The free T4 estimate had been elevated artefactually by the increased affinity of FDH albumin for the analog in a one-step assay. The T3 and T4 concentrations were increased by pregnancy and T4 was increased further by FDH. This first report of FDH recognized during pregnancy emphasizes that the effects of pregnancy on thyroid hormone and TSH concentrations complicate the diagnosis of FDH. It is particularly important to distinguish this benign condition from thyrotoxicosis during pregnancy, because inappropriate treatment may affect fetal development.