"I've been really, really sick since early Sept," Johnston wrote. "Full body muscle weakness to the extent that just walking difficult, stairs impossible and I even need a neck brace to hold my head up."
Johnston explained that she missed two episodes of her TV Land series while she was at the Mayo Clinic to recover. "The good news: I'm finally on the right treatment, feeling better & back at work!"
In her absence, Leah Remini was brought in to replace her. "Thank GOD for Leah Remini!! Her first episode airs Jan. 1st, and it's truly hilarious, I've seen it," she continued. "Seriously, you guys will laugh you're a--es off at her as Stuart's sister. I promise, you won't even miss me! (Well, maybe a teeny tiny bit?)"
Johnston's diagnosis comes seven years after the Emmy-winning 3rd Rock From the Sun alum had part of her stomach removed and spent two months in the hospital — the result of a burst ulcer.