Fact 1: Thyroid hormones increase the rate of metabolism in cells throughout the body.

Fact 2: Cell metabolism is a heat production.

Given the two facts above, answer the question "Thyroid hormones increase the rate of what in cells?" with the following options: 
- kinetic energy
 - heat production
 - heat sources
 - Chemical energy
 - heat energy
 - heat improvement
 - energy usage
 - heat molecules


 

  heat production       