An effective thank you note to a nurse should be personal and heartfelt. Whether the nurse has provided care for the writer or for the writer's friend or family member, the letter should contain words of thanks and appreciation.
It is often appropriate to reference the specific event the nurse is being thanked for, even if it is ongoing. For example, the writer might thank his/her nurse for their diligence and care while recovering from an operation or for their help during a parent or child's recovery process in the hospital. Referencing the event specifically might be helpful for nurses who work in larger hospitals or who maintain more patients under their supervision.
If the nurse cared for someone other than the writer, it is often customary for the writer to include a quote or phrase that has come directly from the patient to the nurse. For example, the writer might express that his/her mother once remarked that "my nurse made being in the hospital feel just like I was at home." A direct quote or reference like this can go a long way in making a nurse feel appreciated for the extra effort they put into their patients' care and well-being.