The actress and comedian revealed this news, just two weeks after responding to people who had been commenting on her "puffer" face online.
She had previously put her changing face shape down to endometriosis, having been making media appearances to promote her new TV series Life & Beth.
Cushing's Syndrome is caused by having too much of the hormone cortisol in your body, and can be serious if not treated. Symptoms include build up of body fat, skin which bruises easily, as well as a red, puffy face, and skin.
Now, in an interview with the News Noise newsletter, Schumer has shared further details on her health issues.
In a diagnosis which has made her feel "reborn", Amy Schumer revealed that she underwent tests which involved being "in MRI machines four hours at a time [and] having my veins shut down from the amount of blood drawn".
Saying that it is the type of syndrome which "will just work itself out", Amy Schumer added, "It has been a crazy couple weeks for me and my family. Aside from fears about my health, I also had to be on camera having the internet chime in".
The comedian continued, "But thank God for that, because that's how I realized something was wrong".
She also explained that the whole situation is "a good example of the fact that we never know what is going on with someone".
Cushing's Syndrome can result from the use of steroid medicines, which contain a synthetic version of cortisol. Amy Schumer revealed that her diagnosis has emanated from receiving steroid injections in high doses.
The comedian's previous health issues include endometriosis, and Lyme disease.
