What should a doctor write for immigration in Thailand if I am hospitalized with Dengue fever before my SETV expires?

I am an in-patient in hospital with Dengue on an IV. My SETV finish on 12 (2 days). The doctor says he is happy to write whatever he needs to inform immigration, as currently I am only getting worse. He told me to find out is there a form he needs to use or anything specific he needs to write. My local immigration is Samui. I am a UK passport holder. TIA