The thyroid gland, through its hormonal activity, plays a central role in regulating human metabolism. Gabriella Morreale, working at the Spanish National Research Council (CSIC), has made pioneering contributions to understanding the role of thyroid hormones during critical stages of development. Her research ranged from elucidating the fundamental mechanisms of thyroid hormone action to uncovering their crucial impact during pregnancy on fetal brain development. When Morreale began investigating this field, it was widely believed that the placenta acted as an impermeable barrier to maternal thyroid hormones. She demonstrated, however, that the placenta is permeable to maternal thyroxine (T4), providing the fetus with essential thyroid hormones during early gestation, before its own thyroid becomes fully functional. This discovery reshaped the understanding of fetal neurodevelopment and highlighted the importance of maternal thyroid health in preventing neurodevelopmental impairments. Her groundbreaking work opened new avenues for perinatal endocrinology, offering insight into neurocognitive outcomes in children and informing strategies for the early detection and prevention of thyroid-related developmental disorders.