Why does our body regulate metabolic processes using the thyroid hormone?
The answer is in special situations, where a trade-off has to be done between thermogenesis and energy saving. This applies to hibernation of animals as well as to critical illness in humans.

This kind of adaptation is referred to as TACITUS syndrome (thyroid allostasis in critical illness, tumours, uraemia and starvation), NTIS (non-thyroidal illness syndrome) or ESS (euthyroid sick syndrome). In this situation, total and free T3 are downregulated, and, in special cases, TSH and T4, too. See an open review article by Wajner and Maja, which is available from http://dx.doi.org/10.3389/fendo.2012.00008 , for details.