Johnston, 46, recently learned that she has "a rare autoimmune disorder" that left her immobile. "My immune system has decided to go rogue [and] attack my muscles," said Johnston.
"Lying down I'm fine, which is great for writing or killing time on social media," the two-time Emmy winner admitted, "but it's kind of disastrous when you're in a sitcom that requires tons of physical comedy."
During four months of tests to find out the cause of her illness, Johnston only took a two-episode break from The Exes where Leah Remini was brought on to carry the load.
Since the diagnosis, doctors have been able to treat Johnston, getting her healthy enough to return to work.
Johnston closed out her social media post, thanking her TV Land family for their support through her ordeal, writing, "It's true that when you're sick, you find out who your real friends are."