Heat illness is a common presentation to emergency departments during periods of high ambient temperatures. The thyroid axis is involved in thermoregulation, and its dysfunction dysfunction leads to loss of thermal homeostasis. Hyperthyroidism predisposes an individual to heat illness and hypothyroidism to hypothermia. For heat illness to be the presenting feature of hypothyroidism is very rare. In this report, a case is presented and a discussion of the thyroid axis, thermoregulation, its failure and possible mechanisms follows on.