The insurer said in October that underfunding of Medicare Advantage plans for the elderly could not be fully offset by the company's other healthcare business. The company also reported spending more healthcare premiums on medical claims in the third quarter, due mainly to government cuts to payments for Medicare Advantage services.

The Journal report said that doctors in at least 10 states were notified of being laid off the plans, some citing "significant changes and pressures in the healthcare environment." According to the notices, the terminations can be appealed within 30 days.

Tyler Mason, a UnitedHealth spokesperson, was not immediately available for comment when reached by Reuters.

The insurer told the WSJ that its provider networks were always changing and that it expected its Medicare Advantage network to be 85 percent to 90 percent of its current size by the end of 2014.