Write your name and address in the top right of the letter, the date of the letter's composition below the last line of your address and the name of the medical director and the address of the hospital below the date. This makes up the standard business letter heading.
Address the director with a semiformal greeting such as "Dear Director Smith" or "Dear Susan." As your letter's primary purpose is to express appreciation, it is not necessary to maintain a formal tone throughout.
Explain the purpose of your letter in the opening line. For example, you might write, "I am writing to thank you for the many great years we spent working together."
Focus on one or two experiences you've had with the medical director in the body of your paragraph that demonstrate the attitude or behavior he possessed that made you so appreciative. For example, you might write, "Your easygoing management style truly helped me as I worked to complete projects," or "Your sense of humor made for a truly relaxed, but dedicated work environment."
Close your letter by expressing your deepest appreciation of your director's attitude or behavior. For example, "I am truly appreciative of all that you have done for me over the years, and I wish you the best in the years to come."
Sign the letter using an expressive valediction such as "Respectfully," or "Warm Regards," before signing your name. If your director is not likely to understand your signature, you may also print your name under your signature.