The role of thyroid hormone in the human fetus is uncertain; a significant amount of T4 is transferred from the maternal to the fetal circulation. A mother-infant pair was found to be heterozygotic for a point mutation in codon 271 of the gene encoding Pit-1, a pituitary-specific transcription factor regulating somatotrope, lactotrope, and thyrotrope function. At birth, serum T4 was undetectable in mother and infant. The newborn presented with a striking delay of respiratory, cardiovascular, neurological, and bone maturation. Despite replacement therapy since birth, neurological development of the infant is impaired. Fetomaternal Pit-1 deficiency resulted in unmitigated fetal hypothyroidism that unmasked thyroid hormone as a potent endogenous drive of fetal maturation and revealed placental transfer of maternal T4 as a rescue mechanism for infants with congenital hypothyroidism, preventing fetal and neonatal symptoms of thyroid deficiency and safeguarding developmental potential.