In pregnant C57BL/6NCrl mice, the combined suppression of adipose thermogenesis, attenuation of central hyperthermia, and redirection of metabolic signaling via FGF21 constitute a hierarchical adaptation that prioritizes fetal protection over maternal thermogenic responses to thyroid hormone.

During pregnancy, the body stops using fat and muscle to generate heat in response to thyroid hormone, and instead uses the liver to release a hormone called FGF21 that shifts energy use toward feeding the fetus. The brain also lowers its temperature setting so the mother doesn't overheat, even though thyroid hormone levels are high. This ensures the baby stays cool while the mother gets enough energy without producing excess heat.