If you would like to thank your colleague or another doctor for referral, here is a simple template you can use to write a thank you letter for referral of a patient.
I would like to thank you from the bottom of my heart for referring one of your patients. I know how difficult it can be to have a lot of patients at the same time. Good thing, my schedule has freed up in the past few days. I was also busy last week and I know how it felt. I immediately found out what to do with your patient. It would be better to tell you in person though. Thus, I would like to invite you to come to my office anytime you wish and I will explain what I found.