M. Spencer Green/AP Photo

Illinois Congressman Jesse Jackson Jr., who took a mysterious leave of absence from Capitol Hill last month is being treated for a "mood disorder," his physician said Wednesday.

"The Congressman is receiving intensive medical treatment at a residential treatment facility for a mood disorder. He is responding positively to treatment and is expected to make a full recovery," according to a statement released by the Democratic lawmaker's office on Wednesday.

"In addition, the rumors about him being treated for alcohol or substance abuse is not true," according to the statement. Jackson, the source of a recent House ethics investigation, took an unannounced leaved absence in June, citing "exhaustion" as the only cause. In the week's since, his fellow lawmakers including fellow party members have called on him to explain the reasons for his absence.

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Today, House Speaker Nancy Pelosi, D- Calif, said "I feel sad that he finds himself in that situation and that he's away from Congress." She noted that he only missed 11 working days of the summer legislative session. Jackson, a Democrat from Chicago first elected to Congress in 1995, is running for re-election this year.

Calls to his campaign to determine the status of his race were not immediately returned. The House Ethics Committee is investigating allegations that Jackson offered to donate money to then Illinois Gov. Rod Blagojevich's gubernatorial campaign in exchange for the Senate seat vacated by President Obama.

Jackson is also being investigated for allegedly directing Raghuveer Nayak, a political donor and friend, to pay for air fare and travel accommodations for a woman Jackson calls a "social acquaintance."

Blagojevich was found guilty of corruption and began serving a jail term earlier this year.

Jackson has denied the accusation.

A witness in Blagojevich's 2010 corruption trial linked Nayak to Jackson's alleged attempt to buy Obama's seat from Blagojevich. According to a Jackson spokesman, no additional information regarding his medical care will be released pursuant to federal privacy laws.