Hyperuricemia is a case of an excessive concentration of uric acid in the blood. It is mainly due to two causes: excessive dietary intake of uric acid, or a defect in the elimination of this acid, mainly in patients with a renal insufficiency. The main consequence of hyperuricemia is the formation of crystals in the joints, and the resulting disorder is called gout. Gout is the sudden onset of pain in the affected joints, often the big toe. If hyperuricemia is not treated by changes in diet, deposits may also occur in the skin: this is called gouty tophi. Some medications can lower blood uric acid concentration such as allopurinol.