(a) The retirement board may, and upon the application of anyone retired pursuant to section 21, 24, or 67a shall, require anyone retired under section 21, 24, or 67a who has not attained age 60 years to undergo a medical examination. The retirement board shall not require a person retired under section 21, 24, or 67a to undergo more than 1 medical examination in any calendar year. The examination is to be made by or under the direction of the medical advisor at the retirant’s place of residence or other place mutually agreed upon. Should anyone retired under section 21, 24, or 67a who has not attained age 60 years refuse to submit to the medical examination, his or her disability retirement allowance or supplemental benefit provided for in section 67a may be discontinued until his or her withdrawal of the refusal. If the refusal continues for 1 year, all rights in and to his or her disability retirement allowance or supplemental benefit provided for in section 67a may be revoked by the retirement board. If upon the medical examination of a person retired under section 21, 24, or 67a, the medical advisor reports and his or her report is concurred in by the retirement board, that the person retired under section 21, 24, or 67a is physically capable of resuming employment, he or she shall be restored to active service with the state and his or her disability retirement allowance or supplemental benefit provided for in section 67a shall cease.

    (b) If the secretary reports and certifies to the retirement board that a person retired under section 21, 24, or 67a is engaged in a gainful occupation paying more than the difference between his or her disability retirement allowance and his or her final compensation, and if the retirement board concurs in the report, then his or her retirement allowance shall be reduced to an amount which together with the amount earned by him or her shall equal his or her final compensation. Should the earnings of the person retired under section 21, 24, or 67a be later changed, the amount of his or her retirement allowance shall be further modified in like manner.