There are basically 2 types of health insurance coverage available in Thailand, government Social Security Insurance (SSI) and non-government, privately funded health insurance.
For expats to qualify for government SSI insurance, they have to work for an employer who offers that coverage to their employees. For one reason or another, though, many employers don't tend to offer it.
For most expats, the only health insurance option for them here is some form of privately funded Thai baht health insurance, like AIA, Luma, Pacific Cross, Aetna, or other health insurance companies. Some life insurers are also authorized to sell a form of limited basic hospital inpatient health insurance coverage.
In any case, all health insurance sold to expats (and Thais) in Thailand is tightly regulated and controlled by the government Office of Insurance Commission (OIC). It may take months for the OIC to approve the selling of an insurance plan in Thailand and many more months to get them to approve any future changes to that plan.
Other than providing tight regulation and control over the Terms and Conditions of the insurance policy and the tight monitoring of an insurer's financial solvency, the government pretty much leaves the insurance companies alone when it comes to medical underwriting and claims processing.. The insurance underwriters can choose whom to insure, what to exclude and whom to reject. The government approved policy Terms and Conditions, though, plays a major role in the approval or rejection of claims
Unlike governments in many other countries around the world, the Thai government does not give the insurance companies any money or any other incentives to encourage them to accept substandard health risks or to cover pre-existing conditions. But, they closely monitor the financial stability of the companies and make certain that they maintain enough capital reserves to handle future claims.
With no financial help from the government forthcoming, the insurance companies must keep themselves financially fit enough to handle any future claims on their own. If they don't, the government can step in and close them down. This has happened with some auto insurers and at least one life insurer.
One of the ways that an insurance company can protect their financial solvency is through the hiring of well qualified Insurance Underwriters with strong medical knowledge. The Underwriter's job is to carefully screen each application to look for, and exclude from coverage, any potential health problems that could become a costly claim in the future.
Finding and weeding out claims that the insurance company shouldn't be responsible for covering is another way that the insurance companies protect their financial solvency. Since not everyone is completely honest in completing insurance applications, the law protects health insurers by giving them an additional two years to find and exclude coverage for health problems not mentioned on the application.
One of the things that the insurance companies watch for during the first two years is whether the condition currently being treated existed at the time of application and wasn't mentioned on the application. If the claims person has reason to believe that the problem did exist at the time of application, the insurance company can refuse to direct pay to the hospital and have the insured pay the bill while the claims person takes more time to carefully evaluate the claim. Once their policy has been in force for more than two years, this should no longer be a problem for insureds.
Another way that health insurers protect themselves financially is by raising the premium for individual insureds with high claims. Generally, if your claims are equal to your premium, or higher, expect a premium increase. How much of an increase depends upon how high your claims were for the past year. 50% increase is common for high claims. I have never seen 100% increases, but I guess it is possible. One insurer limits the individual increases to a maximum of 25%. But, that same insurer has across the board premium increases more often than other insurers.
The final solution for keeping an insurance company financially healthy is to raise the premiums across the board for every insured. Most insurance companies don't like to do that unless they really feel they have to.
Keep in mind that the health insurers based in Thailand are only allowed to sell to residents of Thailand. Some of them may have international names, but they are licensed Thai companies and, therefore, are selling in a much smaller market space than their much larger cousins in Europe or America.