People with hypothyroidism do fine when taking medication containing their daily dose of thyroid hormone. This means that the signaling function of the thyroid hormones is, in principle, unnecessary. If a constant signal would do, then there is no need for that signal in the first place, as you could just by default switch on whatever process is now switched on by the thyroid hormone. So, what's the whole point of regulating metabolism via the thyroid hormone, especially considering that it predisposes us to thyroid disease?
$\begingroup$ Because it evolved that way, and there was not a significant evolutionary pressure to change it? And possibly not a mechanism to allow the change, once humans' distant ancestor had evolved a proto-thyroid. $\endgroup$– jamesqfMar 12, 2016 at 18:27
1$\begingroup$ @Count I disagree slightly with "...the signaling function of the thyroid hormones is, in principle, unnecessary..." Thyroid hormone (T4 - aka "Thyroxine") has a half-life of 4-5 DAYS. It is essentially a constant level in the circulating plasma that is important - this is why patients with hypothyroidism that take exogenous T4 do fine. It doesn't have anything to do with T4 not being important biologically. If you take exogenous thyroid hormone away from someone with hypothyroidism or who has had their thyroid removed surgically, once their plasma T4/T3 is gone they do horribly. $\endgroup$– Vance L AlbaughJul 7, 2016 at 0:55
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.