It is clear that thyroid hormones modulate non-shivering heat production of the newborn mammal. While some obvious thyroid hormone effects on thermogenesis can be demonstrated in adult, cold-acclimated or hibernating animals, these findings cannot be directly applied to the newborn, in which changes in amount and composition of brown adipose tissue, as well as sympathetic and thyroid system maturational events, are occurring. Thyroid hormones do influence the prenatal development and subsequent responses of brown adipose tissue in neonates through primary actions on brown adipose tissue iodothyronine 5'monodeiodinase and mitochondrial uncoupling protein. Secondary effects involving brown adipocyte growth, lipid composition, oxidation proteins, and sympathoadrenal activity are of lesser importance in thyroid hormone modulation of newborn thermogenesis. The acute surge in thyroid hormones that occurs at birth seems of limited significance with regard to neonatal thermogenesis. The stimulation of sympathoadrenal activity at the time of birth, as well as continued in situ conversion of T4 to T3 in the brown adipocyte, are of critical importance in stimulating and modulating heat production in the newborn.