Swelling of the legs may be caused by renal failure, hypothyroidism, lower extremity venous thrombosis, etc. Swelling of the legs may also be seen in heart failure, cirrhosis and other diseases. 1. Renal failure: renal failure will lead to increased albumin in the urine and decreased albumin in the blood, causing a decrease in plasma colloid osmotic pressure, which will lead to tissue fluid aggregation in the interstitial space of the tissues and cause swelling of the legs. 2. Hypothyroidism: Hypothyroidism affects the metabolic function, leading to excessive mucopolysaccharides that are not metabolized, and the mucopolysaccharides and the tissue fluid in the body will be combined, resulting in swelling of the legs. 3. Lower extremity venous thrombosis: the formation of venous thrombosis in the lower extremities will lead to obstruction of blood flow in the lower extremities, and poor blood circulation will lead to leg swelling. Leg swelling can also be seen in heart failure, cirrhosis of the liver and other diseases, if the symptoms of leg swelling, you need to consult a doctor in a timely manner to check, according to the cause of the disease to take appropriate treatment.