Tenn. Code § 8-4011
Persons electing membership in the retirement system created by this chapter shall not hold membership in any other public employees' retirement system, except that coverage by social security and membership in the Tennessee teachers' retirement system are specifically permitted; provided, however, that such persons shall be entitled to enjoy, unimpaired, any vested benefit existing under such other system, but in such case, no periods of time on account of which such vested benefits exist shall be considered credited service under this chapter.
Any member of the Tennessee retirement system for county officials who also holds membership in any other public employees' retirement system may elect to withdraw his membership in full and his contributions in full from such other system without impairing his right to hold office in the county concerned and receive credited service under this chapter in the Tennessee retirement system for county officials for such credited service as he held in such other system upon applying for membership in accordance with § 8-4012.
T.C.A. § 8-4011