FIRST ON FOX: Mystery is swirling around Pennsylvania Democrat Senator John Fetterman as he performs his senatorial duties while hospitalized for clinical depression.

Fetterman this week co-sponsored rail safety legislation, despite currently being hospitalized. 

New York University medical professor Dr. Marc Siegel told Fox News Digital that, while we should accept depression as an illness and encourage treatment, he finds it "very unusual" Fetterman is continuing his course of work from the hospital.

Siegel, a physician by trade, said what "we don’t know" is how "severe" Fetterman’s depression is, what kind of therapy he is getting, and how he is responding, and that Fetterman’s course of stay in the hospital is the normal procedure for someone suffering from clinical depression.

DEMOCRAT SEN. FETTERMAN’S OFFICE RELEASES UPDATE AFTER HE CHECKS HIMSELF INTO HOSPITAL FOR CLINICAL DEPRESSION

John Fetterman

New York University medical professor Dr. Marc Siegel told Fox News Digital that, while we should accept depression as an illness and encourage treatment, he finds it "very unusual" Pennsylvania Democrat Senator John Fetterman is continuing his course of work from the hospital. (AP Photo/J. Scott Applewhite, File)

Siegel noted that "mental illness and physical illness aren't the same in terms of the level of impairment."

"So if you were to say to me, somebody had a heart attack, they're still in the hospital, can they be a senator? You know, my answer would be depends on how their heart is doing right now," Siegel said.

"And, in general, the answer would be no. That if you're in the hospital for severe depression, that your judgment is impaired, and that your ability to think clearly and rationally is impaired," he continued. "But we don't know that."

Siegel said he wants "to know what his psychiatrists think" with Fetterman’s recovery but noted that it’s "very unusual that somebody that's being admitted for severe depression would be performing their job."

Dr. Marc Siegel

Siegel said that he is "very surprised" that Fetterman is continuing to do his job as a senator, joining caucuses and co-sponsoring legislation, adding he "would be very disturbed and concerned about the idea of him performing the job of senator while receiving intensive treatment for severe depression." (Fox News)

"That’s very unusual," Siegel said. "Let alone as a senator."

"If you broke your ankle, you know, nobody would think twice. Okay, he's got surgery on his ankle and he's recovering, but he's performing from the bed. But mental illness, by definition, is an impairment of mood and judgment. So it would be highly, highly unlikely that somebody could do that, unless they're just keeping them there to kind of keep them out of the way of the stress of daily living, but they feel that he's up to the job."

"Again, highly unusual," Siegel added.

Siegel said that he is "very surprised" that Fetterman is continuing to do his job as a senator, joining caucuses and co-sponsoring legislation, adding he "would be very disturbed and concerned about the idea of him performing the job of senator while receiving intensive treatment for severe depression."

Fetterman pointing

PITTSBURGH, PA - NOVEMBER 09: Democratic Senate candidate John Fetterman speaks to supporters during an election night party at StageAE on November 9, 2022 in Pittsburgh, Pennsylvania. Fetterman defeated Republican Senate candidate Dr. Mehmet Oz.  (Jeff Swensen/Getty Images)

"Those two don't go together," Siegel said. "If, however, he's responded to treatment for severe depression and they're just keeping an eye on him for a few days, that is more reasonable."

Siegel also speculated that the "best-case scenario" for Fetterman is that his "doctors feel that he's back up to a level of where he's no longer severely depressed."

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"But they're afraid to let him out because they don't know how he'll do on the outside," Siegel said. "So they're keeping an eye on him while allowing him to work to restore his self-esteem, but they don't feel he can be alone."

Fetterman's health has been a subject of scrutiny after his stroke while campaigning against Dr. Mehmet Oz for the seat.

Joe Calvello, Fetterman's spokesperson, told Fox News Digital that "John is getting briefed daily by his staff and is in communication with his colleagues in the Senate."

"As we have said multiple times this will be a weeks-long process," Calvello said.