The thyroid hormones T3 (triiodothyronine) and T4 (thyroxine) functions to increase the overall basal metabolic rate of the body. In the absence or decreases of thyroid hormones (i.e. iodine deficiency, late stage Hashimoto's thyroiditis) then the overall basal metabolic rate of the body will decrease and will lead to cold intolerance (a key symptom in hypothyroidism) which will decrease the temperature at which shivering (thermogenesis) begins.