Final answer:
Antipyretics are ineffective in treating hyperthermia caused by acute brain injury due to the dysfunctional thermoregulation from the brain rather than a fever response initiated by the hypothalamus.
Step-by-step explanation:
Antipyretics are not effective in treating hyperthermia in cases of acute brain injury because hyperthermia in this context is not caused by the reset of the hypothalamic set point, which antipyretics target, but by a failure of the body's heat dissipation mechanisms or excessive heat production. Hyperthermia due to brain injury can result from damage to the hypothalamic thermoregulatory centers, leading to an inability to effectively manage and dissipate body heat.
In cases of infection, the body deliberately increases its temperature as a defense mechanism by releasing pyrogens which reset the hypothalamic thermostat to initiate a fever. However, hyperthermia in acute brain injury does not involve this pyrogen-mediated fever response, and thus, antipyretics, which normally lower body temperature by acting on the hypothalamus, are ineffective. Instead, the elevated temperature is due to the body's inability to regulate temperature due to neural dysfunction or external factors like the environment.