First off I'd go into my health insurance portal and change my password. Then use the forgot password, and see if they are still mailing your password in plain text. Do a bit of investigation to confirm that all passwords are still stored in plain text.
Once you can confirm that your password is sent in plain text, I'd contact the insurer to make sure they are aware of the security implications.
If you've read Troy Hunt at all, take a book out of his practice. They probably won't make any change, or understand, but you've tried to help.
Then, change insurance companies if you fear your data is at risk, which it probably is.