Hospital Fees Massive Rise When Health Insurance Used

Actually, the insurance industry in Thailand is regulated, and the regulating body (OIC) has an arbitration unit to which you can bring disputes. http://www.oic.or.th/en/weblink/index.php

However it is true that the regulations are not as restrictive as they are in many countries (for example, companies are allowed to increase your premiums by 10 - 25% if you develop a condition that changes your "risk profile", not the case in most Western countries.

And, the OIC's main focus is not on private health insurance, which so very few Thais have, but rather things like vehicle insurance.