Question about using the hiring manager's first name in a thank you letter?

I interviewed with a company today and the interview went so well that they almost instantly decided to ask me back for a 2nd interview on Tuesday.





I'm working on a thank you letter to the hiring manager and I'm planning to address the manager by her first name (in the letter).





My reasoning for using her first name is that they are on a first name basis in the office.





What do you think? Should I be using her first name in the letter or her last name?

Question about using the hiring manager's first name in a thank you letter?
I don't think that is proper. Perhaps it is fine if you are in conversation with him, but if you want to properly thank him, I'd use his last name.





Best to error on side of caution.
Reply:Maybe consider setting it up in a memorandum format in which the 'To' section has her full name and work address. Then open it with Dear 'First Name'... Next paragragh, I would like to take the opportunity to extend my appreciation... etc.
Reply:Last name only like Ms. Henley or Mrs. Henley. They may use first names after you are part of the team but right nor be professional.
Reply:I guess that would depend on how she introduced herself to you at the beginning of the interview. If she said, "Hi, I'm Mrs. Jones" then that is how you should address the letter. However, if she said "Hi, my name is Susan" then by all means use her first name.
Reply:Hi - the best protocol for this is to keep it respectful. If you are typing a letter, address it to Dear Mr. Doe but then strike that out with your pen and handwrite "John".