In this subchapter, "insurer" means:
(1) an insurance company, including a company providing stop-loss or excess loss insurance;
(2) a health maintenance organization operating under Section 1367.053, Subchapter A, Chapter 1452, Subchapter B, Chapter 1507, Chapter 222, 251, or 258, as applicable to a health maintenance organization, Chapter 843, Chapter 1271, and Chapter 1272;
(3) an approved nonprofit health corporation that holds a certificate of authority issued under Chapter 844; or
(4) a third-party administrator that holds a certificate of authority under Chapter 4151.