A treatment fluid for treating a portion of a high-temperature subterranean formation comprises: a base fluid, wherein the base fluid comprises water; a viscosifier, wherein the viscosifier is a polymer, and wherein the viscosifier is thermally stable up to a temperature of 325° F.; and a stabilizer package, wherein the stabilizer package: (A) comprises a first stabilizer and a second stabilizer, wherein the first stabilizer is an oxygen scavenger and the second stabilizer is a pH adjustor; and (B) is capable of increasing the thermal stability of the viscosifier to a temperature greater than 350° F., wherein the portion of the subterranean formation has a bottomhole temperature greater than 350° F. A method of treating a high-temperature subterranean formation comprises introducing the treatment fluid into the portion of the subterranean formation.