The hypothalamus compares core temperature with a set point and drives heat loss or gain
Mnemonic
Four routes of heat loss, and one of them behaves differently:
- Radiation - about 60 per cent at rest
- Convection
- Conduction
- Evaporation - about 20 per cent at rest, but the only route that works when ambient temperature exceeds body temperature
“When the air is hotter than you are, sweating is all you have left”, which is why humidity is so dangerous in heat: it blocks evaporation.
The controller is the anterior hypothalamus for heat loss and the posterior for heat conservation, responding to core temperature and to skin thermoreceptors.
Fever versus hyperthermia is the distinction that matters clinically: in fever the hypothalamic set point is raised by pyrogens, so the patient shivers and vasoconstricts to reach it, and antipyretics work. In hyperthermia, as in heat stroke, neuroleptic malignant syndrome or malignant hyperthermia, the set point is normal and heat production simply exceeds loss, so antipyretics do nothing and active cooling is required.
Expansion
The anterior hypothalamus acts as the thermostat, receiving input from central and peripheral thermoreceptors.
Heat loss (when too hot)
- Cutaneous vasodilatation, through withdrawal of sympathetic tone
- Sweating (sympathetic but cholinergic)
- Behavioural change
Heat gain and conservation (when too cold)
- Vasoconstriction
- Shivering, which can increase heat production fivefold
- Non-shivering thermogenesis in brown fat, important in neonates
- Increased thyroid and catecholamine activity
Fever results from pyrogens (interleukin-1, interleukin-6, tumour necrosis factor) raising prostaglandin E2 in the hypothalamus, which raises the set point. The patient then feels cold and shivers until the new set point is reached, which is why rigors accompany a rising temperature. Antipyretics work by inhibiting prostaglandin synthesis.
Hyperthermia (heat stroke, malignant hyperthermia, serotonin syndrome) has a normal set point with failure of heat loss or excessive production, so antipyretics are useless and physical cooling is required.