The first thing is to make sure that the reader is directed towards the suffering of the writer. The amount of suffering that would require him to be out of his duties for a couple of days or months depending on its gravity. Do mention the exact nature of the sickness if there is requirement of long sick leave. Also, it would be advisable to mention that the writer has been performing to his duties with total dedication and also with great results while on normal health days.
There needs to be a direction wherein if the writer ignores the health concern then it might take a toll on his health as well as on his duties with much larger implications considering his role he is having in the office.
Finally, thank the reader for him to approve the leave and his approval is awaited by the writer very soon considering that it is involving health issues.