COLUMBUS - One of Ohio's largest health care providers is leaving the state's health care exchange, leaving 18 counties without an insurance option.

Anthem will pull out as of Jan. 1, 2018, so it won't be a choice for people buying individual insurance later this year. The major insurer said it is uncertain about the future of health care in the U.S., as Republicans in Congress work to overhaul the Affordable Care Act, better known as Obamacare.

Anthem would be the first to leave Ohio's exchange since Aetna announced that it would pull out ahead of 2017.

In 18 counties, including Muskingum and Coshocton, Anthem is currently the only option for Ohioans who have insurance purchased on the exchange. The 18 counties, with 10,500 Anthem insurance customers, are concentrated in Appalachia and include some of the poorest in the state.

Anthem said it was concerned it might lose subsidies that insurers receive to offset the costs of insuring lower-income individuals.

Last month, President Donald Trump reportedly told advisers he wanted to end these subsidies.

“An increasing lack of overall predictability simply does not provide a sustainable path forward,” Anthem spokesman Jeff Blunt said in a statement.

Gov. John Kasich has advocated against congressional Republicans' latest version of an Obamacare overhaul, mostly because it would end federal money for Medicaid expansion. On Tuesday, his administration called on Congress to protect the individual insurance market as well.

“For the past few years we have seen a weakening in the federal insurance marketplace as a number of companies have withdrawn from the exchange," said Chris Brock, a spokesman for the Ohio Insurance Department, in a statement. "We have always argued the private insurance market is the most severely impacted by the federal law and that is where Congressional action is needed to restore stability."

The Insurance Department is working on a solution for the Ohioans in 18 counties who would be without an individual insurance provider as of January, Brock said.

In all, nearly 70,000 Ohioans currently have Anthem coverage they purchased on the exchange.

Anthem will continue to offer insurance in Ohio through small and large employers. It also will continue to cover Ohioans who bought their own plans from Anthem before December 2013, Blunt said in a statement. In addition, the company offers an individual insurance plan to residents of Pike County that is not part of the exchange, and this plan will continue, Blunt said.

Washington correspondent Deirdre Shesgreen contributed to this report.