Primary thyroid failure, usually in patients treated for Graves disease, is the most common reason for hypothyroidism; autoimmune hypothyroidism is the second most common cause. Women are more commonly affected than men. Mild hypothyroidism is accompanied by fatigue, cold intolerance, and skin changes. Hypothyroidism should be considered in patients with chronic constipation, bradycardia, or cardiomegaly. Myxedema coma is a rare but lethal stage of severe hypothyroidism most commonly occurring in patients older than 55 years. Cold exposure is a common precipitating event for the development of hypothyroidism. Although only a small percentage of hypothermic patients have hypothyroidism, 80% of patients with myxedema coma are hypothermic. Physical signs include respiratory failure, abdominal distention, seizures, psychosis, and sinus bradycardia. Laboratory findings include hypercholesterolemia, hypoglycemia, and elevated CSF protein, as well as low serum T3 and T4 levels. Treatment includes intravenous administration of thyroxine and corticosteroids because many of these patients may have adrenal insufficiency. Even with aggressive therapy, the mortality rate is close to 50%.