Iowa Code § 496C.13

No shareholder of a professional corporation shall create or enter into a voting trust or any other agreement conferring upon any other person the right to vote or otherwise represent any shares of a professional corporation, and no such voting trust or agreement is valid or effective. Any proxy of a shareholder of a professional corporation shall be an individual licensed to practice in this state a profession which the corporation is authorized to practice. Any provision in any proxy instrument denying the right of the shareholder to revoke the proxy at any time or for any period of time is not valid or effective. This section does not otherwise limit the right of a shareholder to vote by proxy, but the articles of incorporation or bylaws may further limit or deny the right to vote by proxy.