Thermolabile refers to a substance which is subject to destruction/decomposition or change in response to heat. This term is often used to describe biochemical substances.
For example, many bacterial exotoxins are thermolabile and can be easily inactivated by the application of moderate heat. Enzymes are also thermolabile and lose their activity when the body temperature rises. Loss of activity in such toxins is likely due to change in the three-dimensional structure of the toxin protein during exposure to heat. In pharmaceutical compounds, heat generated during grinding may lead to degradation of thermolabile compounds.