After four months and 17 doctors, she learned the reason why – The Exes star has been diagnosed with "a rare autoimmune disorder."
The actress, who won two Emmys on 3rd Rock From the Sun, shared the news in a post on her Facebook page Tuesday. "It's time for me to be upfront with you about something that's been going on with me," she wrote.
Johnston, 46, revealed that she's been "really, really sick" since early September, suffering from full body muscle weakness that made even walking difficult, "stairs impossible and I even need a neck brace to hold my head up."
"Lying down I'm fine, which is great for writing or killing time on social media," she admitted, "but it's kind of disastrous when you're in a sitcom that requires tons of physical comedy."
Taking a BreakJohnston worked as much as she could through the fall and then spent two "fun-filled weeks in November partying at The Mayo Clinic" where doctors finally diagnosed her illness, which she initially declined to identify.
"My immune system has decided to go rogue [and] attack my muscles," she explained, adding that the good news was that she was finally on the right treatment, feeling better and was back at work."
The actress was forced to take a two-episode break from her TV Land show, explaining that Leah Remini had been brought in for a multi-episode arc to help cover for her.
Remini's first episode airs Jan. 1 and Johnston writes, "You guys will laugh your asses off at her as Stuart's sister. I promise, you won't even miss me! (Well, maybe a teeny tiny bit?)"
Johnston also thanked everyone at the show and TV Land for their support. "It's true that when you're sick, you find out who your real friends are," she wrote.
In Her ShoesAs for her decision not to name her illness, Johnston wrote in an additional post Tuesday, "If I'd wanted you to know, I would've said it. I purposely didn't name the specific auto-immune diagnosis. Please, allow me this time before I have to champion another issue."
By late Wednesday, news of her health issues had spread and Johnston found herself revealing the specific diagnosis.
"I'm not trying to be 'coy,' 'relevant' or 'get attention,' " she writes. "I just was diagnosed two days ago with a rare form of lupus called 'Lupus Myelitis' – Google it. I'm not on my death bed … far from it. I'm gonna be fine."
"I just wanted to have a few weeks of privacy to navigate it and receive treatment. The only reason I said anything is because I won't be in a few episodes," explains Johnston. "I wonder how all those people leaving comments on gossip/entertainment/TV websites would feel in my shoes?"