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Kennedy said he planned to visit Jackson on Thursday. He said he and Jackson had a lot in common: Both served on the House Appropriations Committee together and had famous fathers.

Jackson’s office didn’t announce his medical condition until about two weeks after he went on leave, and it initially described the problem as exhaustion. Later, his office disclosed that Jackson had “grappled with certain physical and emotional ailments privately for a long period of time.” A statement from an unnamed doctor said Jackson had a “mood disorder.”

Earlier this month, Jackson’s office announced he was at Mayo and being treated for depression and gastrointestinal issues, after a transfer from the Sierra Tucson Treatment Center in Arizona.

Though the Mayo Clinic mentioned Jackson’s weight loss surgery, its statement Monday stopped short of directly tying it to his mental health problems. Mayo Clinic spokeswoman Traci Klein declined to comment.

Dr. Jaime Ponce, president of the American Society for Metabolic and Bariatric Surgery, said there is no evidence that the type of surgery Jackson had can cause bipolar disorder. A deficiency of the nutrient thiamine can cause a brain condition that could mimic bipolar disorder, Ponce said, but “bipolar disorder is totally different.”

Jackson underwent a duodenal switch procedure in 2004, which involves removing part of the stomach and rearranging the intestine so less food is absorbed. He lost 50 pounds.

Dr. Vivek Prachand, associate professor of surgery at University of Chicago, said people already taking medications for depression can undergo weight loss surgery but may need their medications adjusted afterward. Prachand added that surgery is a drastic change that can trigger an episode in someone with a history of depression.

Jackson aide Rick Bryant said last week that Jackson appeared in good spirits and wanted him to push forward on projects in the district, which includes Chicago neighborhoods and suburbs. Jackson, who first won office in 1995, is on the November ballot with two little-known candidates and is widely expected to win re-election.

The timing and manner in which the medical leave was handled has invited scrutiny.

Jackson is under a House Ethics Committee investigation for ties to imprisoned former Illinois Gov. Rod Blagojevich. Jackson’s office announced his leave just days after a former fundraiser connected to the probe was arrested on federal medical fraud charges.

Jackson has denied wrongdoing.

Associated Press writers Carla K. Johnson in Chicago and Michelle R. Smith in Providence, R.I. contributed to this report.