When writing a letter, follow business letter format. I plan on checking my email periodically throughout the day but will let you know if my condition worsens and I need to go fully offline. If you are able, it is helpful if you can find a colleague to back you up when you are out in case of emergency.
Occasionally, you might even have to miss a day or two of work.
You do not need to go into all the details of your symptoms. I was unable to attend work due to sickness. Even if it is not required, it is a good idea to tell your boss about your absence as soon as possible.
When writing an email, keep your message just as professional as any business correspondence. Does that work for you? Regardless, make sure you notify your boss as soon as you suspect you'll need to take a day off to help alleviate any logistical challenges.
Learn how to notify your boss about your absence, and read sample excuse letters and emails.
The last thing you need is to be judged for careless mistakes or typos. What to Include in the Letter or Email Use business letter format.
It should inform key colleagues that you will be taking the day off, whether or not you will be active during your sick day and how they can proceed while you are away. Include the same content, but underneath your signature, include your contact information.