Fatigue and cold intolerance are commonly associated with hypothyroidism. Hypothyroidism is when your body has low levels of the hormones made by your thyroid, a gland in your neck. Many diseases can cause your thyroid to function poorly and up to 2 percent of the population have hypothyroidism. Hypothyroidism is also associated with weight gain, constipation, dry skin, muscle aches, changes in your menstrual cycle, and a slow heart rate.