The recent case of Jahi McMath leads me to wonder. . .
Since a lot of our body heat is in a large part is caused by metabolism, and since her brain has shut down, would that also shut down metabolism? *If not, how is her body retaining heat? I could see piling blankets on her body but that would seem to just maintain the ambient temperature.
*Possibly, for all we know, they have already removed life support and disposed of the body, so it’s all irrelevant, but it would still apply to the first three weeks that she was hooked up.
There’s a missing step in your reasoning: The brain isn’t responsible for metabolism. Well, I mean, I suppose there’s the metabolism of the brain itself, which isn’t nothing, but most metabolism is going on at a lower level than the nervous system cares about. I mean, even single-celled things have metabolism.
Body temperature is regulated by the hypothalamus. In brain death, the hypothalamus loses its neural connection to the rest of the body, so temperature regulation breaks down, even if the hypothalamus itself is still intact and able to produce hormones. According to the first reference I found, the result is poikilothermia, an inability to maintain consistent core temperature. Metabolism is still ongoing, but the body can’t tell when it needs to crank it up or where to send the blood, so it’s basically just the waste heat from the cells going about their business. The body temperature drops, especially in the extremities, and blankets can only do so much. From recent reading, I gather that there are other measures to shore up the body temperature–heat lamps, warmed IV fluids, possibly hormone treatments–but it’s an unstable situation.
Search this thread for some excellent posts by WhyNot describing the consequences of brain death.
Short version: “brain dead” means significant portions of the brain are dead, but the rest of the body is not. The brain stops regulating metabolism, but the heart will continue to beat on its own, and the rest of the body will remain alive, as long as you continue to provide oxygen (with a mechanical ventilator forcing air in/out of the lungs) and nutrition (either IV or gastric tube). Bodily metabolism will still produce heat, but the lack of regulation by the brain means that maintaining a reasonable body temperature now requires a lot of attention by hospital staff, adding/removing blankets and/or cooling, depending on what’s required to keep the core temp somewhere near 98.6F.
The hypothalamus regulates the pituitary gland’s secretion of thyroid stimulating hormone. No hypothalamus = no TSH secretion by the pituitary = poorly-functioning thyroid gland = wonky low basal metabolic rate.
This can be offset to a degree by administering exogenous thyroid hormones like Synthroid, but it’s hard to get the levels right (especially since in a living person you measure serum TSH levels to see if the Synthroid dose you’re giving is working).