Arthur Gianelli, the chief executive of NuHealth, which runs Nassau University Medical Center and other facilities that serve the poor, has abruptly announced his resignation.

Gianelli, credited with putting the system in a strengthened fiscal position, sent a three-page letter to staff on Friday saying he was "not sure where I will end up" and that he would stay on the job "for at least the next three months."

No explanation was given for his resignation, but sources said they believed he was forced out.

Gianelli declined to comment Monday.

He was appointed to the post in March 2006 by then-Nassau County Executive Thomas Suozzi, a Democrat who last week lost a rebid for the office to incumbent Edward Mangano, a Republican.

Since 1999, Nassau Health Care Corp., known as NuHealth, has received $18 million a year from the county. The money generally covers uncompensated care and some public health expenditures. Of that, $13 million is slated to be discontinued in 2015.

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NuHealth, which is made up of NUMC in East Meadow, the A. Holly Patterson Extended Care Facility in Uniondale and five clinics, employs about 3,500.

Craig Rizzo, chairman of the NuHealth board of directors, said in a statement that the board "will be conducting a nationwide search" for a new chief executive. The 18-member board, three of whom are nonvoting, is named by the county executive, county legislature or by the governor, based on their recommendations.

Gianelli's resignation comes two weeks after Gov. Andrew M. Cuomo signed into law a closer collaboration between NuHealth and North Shore-Long Island Jewish Health System. The agreement allows NuHealth to negotiate alongside North Shore-LIJ with commercial insurance companies and continue expanding the sharing of doctors and services.

NuHealth board member Dr. George Miner said Gianelli's resignation "came out of the blue. I can't imagine what would have happened." Board member Greg Martello said he hoped the board tries to keep Gianelli.

NuHealth's chief medical officer, Dr. Steven Walerstein, said that under Gianelli "we've seen significant improvement in quality of patient care, in the academic and training program and in the physical facilities."

Civil Service Employees Association Local 830 president Jerry Laricchiuta, with whom Gianelli had long disagreements over union contracts, said of the resignation: "I don't think it's a good thing." Despite "years of arguments and tensions and grueling negotiations, I think he had the same goal as we do and that was to make it the best hospital," Laricchiuta said.

North Shore-LIJ chief executive Michael Dowling praised Gianelli for "restoring financial stability to this safety net hospital while enhancing the quality of care." Despite Gianelli's exit, he said, "we will continue to work in partnership with NuHealth."

Two years before Gianelli took the job, a consultant said that "by virtually any measure, NHCC today stands on the brink of financial collapse."

Although still financially vulnerable, the corporation has since balanced the budget three times and Gianelli said in his letter to the staff that NuHealth had brought in more than $200 million in government and private funding.

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NuHealth highlights

1935: Meadowbrook Hospital, with 582 beds, opened in East Meadow.

1974: Nassau County Medical Center, a county-run hospital, opened on same site with 725 beds.

1997: Nassau Health Care Corp., a public-benefit corporation created by the State Legislature, was established. This removed the hospital, A. Holly Patterson Extended Care Facility and five health centers from county control.

2001: Hospital was renamed Nassau University Medical Center.

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2004: Consultant identified NHCC as on brink of financial failure.

2006: Arthur Gianelli became chief executive.

2010: NHCC also became known as NuHealth.

Sources: NuHealth, Newsday