Applegate first found fame as a child actress on US sitcom Married... With Children and won an Emmy for playing Rachel's sister Amy in Friends.
"It's been a strange journey. But I have been so supported by people that I know who also have this condition," Applegate tweeted on Tuesday.
She asked for "privacy" as "I go through this thing".
The Dead To Me actress added: "It's been a tough road. But as we all know, the road keeps on going. Unless [someone] tries to block it."
