What happens to energy used for maintaining body temperature?

Energy used for maintaining body temperature is ultimately released as heat.

To maintain a stable body temperature, the body adjusts the rate of heat production (by increasing or decreasing metabolic rate) and the rate of heat loss (by adjusting the dilation or constriction of blood vessels in the skin, allowing for more or less blood flow and heat transfer). When the body produces or absorbs heat faster than it can be dissipated, the core body temperature changes accordingly.

Any energy that is used for cellular functions, including those involved in maintaining body temperature, is ultimately converted into heat. According to the laws of thermodynamics, energy cannot be created or destroyed, but it can be transferred and transformed. In this case, the energy used for cellular processes ends up as thermal energy (heat) that is either released into the environment or used for other purposes in the body.