Here's an example of what you could write:
Request for sick leave letter to send to your boss:
Dear Sir/Madame
I am writing to notify you of a serious bout of ill-health I have been experiencing.
After seeking the appropriate medical advice and consulting with my doctor, I have been advised to request a period of absence in order to recuperate.
The length of my recuperation is expected to be [how long you need off] and I will of course keep you informed of any changes in my condition as soon as possible.
I am also able to provide medical documentation if you so require them, so please do not hesitate to ask.
I am sincerely sorry for any inconvenience this may cause, and I look forward to returning stronger and more keen than ever.
Best Regards,
Your name.
