Non-shivering thermogenesis occurs in two settings in Man:
term neonates: brown fat in the region of the shoulders and back is metabolised under adrenergic stimulation. Adrenaline and noradrenaline stimulate uncoupled oxidative phosphorylation to produce heat rather than ATP. Neonatal hypothermia may occur if brown fat is lacking e.g. in prematurity.
adults: an increase in the basal rate of metabolism in a variety of tissue e.g. liver, intestine and skeletal muscle. The hypothalamus is believed to integrate the response by:
producing TRH, ultimately leading to increased T4 and T3 synthesis and release
increasing sympathetic outflow from both noradrenergic nerves and stimulation of adrenaline production in the adrenal medulla