How does an endotherm respond to a decrease in external temperature?

What's an endotherm? An animal that can increase its body temperature by metabolically produced heat.
Why does a decrease in temperature matter? 37ºC is the optimal temperature for enzyme function – endotherms need to maintain this internal temperature to ensure metabolic reactions occur at the optimal rate for survival.
How is this change detected? Peripheral thermoreceptors in skin stimulated by a decrease in temperature, sending impulses to the hypothalamus. The hypothalamus also directly detects changes via its thermoregulatory centre.
What can the endotherm do about it? Behavioural and physiological responses to return temperature to optimum (negative feedback).
Behavioural responses include moving into the sunlight, hiding in a burrow, or increasing your level of activity, since a lot of energy is lost as heat. These are often initiated when changes are detected by peripheral thermoreceptors as a feed-forward mechanism: the core temperature might not have dropped yet, but it will do if we don't do anything about it!
Physiological responses include (a) reducing heat loss and (b) increasing heat generation:
raising the hairs on the skin using pili erector muscles to trap a layer of insulating air (air has a low conductance)
vasoconstriction of arterioles and superficial capillaries using sphincter muscles – this reduces conductive heat transfer by increasing the distance between the blood and the environment (think increasing x in Fick's law!)
liver cells increase the rate of metabolism to generate heat energy
shivering is the spontaneous spasm of the skeletal muscles to generate heat
hormonal mechanisms: thyroid gland releases thyroxine to increase metabolic rate, again increasing metabolic rate