Hypothyroidism occurs when you have low levels of thyroid hormone in your body, usually due to problems with your thyroid, a gland in your neck. Thyroid hormone is responsible for stimulating metabolism. Metabolism, in general, is the work done by your tissues to keep you alive and moving at a functional pace. Your body produces heat as a byproduct of metabolism. Thus, lower thyroid hormone leads to lower metabolism, which leads to less heat production and poor tolerance of the cold (other symptoms of hyypothyrodism include fatigue, weight gain, dry skin, thin hair, weakness, constipation, depression and joint pain).