When sending a complaint letter to an insurance company, it can be handy, or necessary, to cc it your state's Insurance Commissioner. The Patient Advocate Foundation has a big list where you can look up insurance commissioner's mailing addresses and phone numbers by state.

Insurance Commissioners are tasked with enforcing state insurance laws and investigating their breach. Sending a copy of your complaint to them, and making sure they're listed as a CC on the letter to the insurance company, can help insurance companies take your letter more seriously.

Insurance Commissioners By State [Patient Advocate Foundation]