Tenn. Code § 17-531
A member entitled to a retirement benefit may elect at any time prior to his retirement date to have a retirement benefit payable under one of the options set forth in § 17-532 in lieu of all the benefits he may otherwise be entitled to receive. The benefit shall be paid in accordance with the terms of the option elected. A member may revoke his election of an option, and he may make a new election at any time prior to retirement. Any election of an option by a member which is not received by the board during the lifetime of the member and in such form and manner which the board may prescribe, or which is not approved by the board, shall be null and void. The board may disapprove any election of an option if the election of said option is deemed to be inconsistent with the pruposes of this chapter. The beneficiary last designated by the member as of the date he delivers written application for an option to the board shall be the beneficiary to receive any benefits payable after his death. The election of an option by a member shall be null and void if the designated beneficiary dies before benefits commence.
T.C.A. § 17-531