In the normal person, body core temperature is tightly regulated around a set value of 37° C ± 0.2° by hypothalamic thermoreceptors, despite fluctuation in heat uptake, heat production, and heat loss. Diurnal variations of ± 0.5° C occur with the minimum value in the morning and maximum in the evening.

In critical care patients, this normal thermoregulation can be compromised by several factors and can cause perturbations of both hyper-and hypothermia. For instance, not every fever is caused by infection. Both exogenous (e.g., bacteria) and endogenous pyrogens can set the “thermostat” at a higher value. Infection, inflammation, and necrosis activate macrophages, which in turn release interleukin 1 and interleukin 6 and increase prostaglandin metabolism. An initial but temporary temperature rise following surgery is classically thought to be part of an inflammatory response. Suspicion for an infective cause is raised by sustained, recurrent, and/or prolonged period of temperature typically >38.3° C.