Adds background throughout

Jan 23 (Reuters) - UnitedHealth Group UNH.N on Thursday named Tim Noel, the head of its Medicare business, to the top job at its health insurance business, a month after the unit's former CEO Brian Thompson was killed.

Noel will take charge at UnitedHealthcare, the largest U.S. health insurer that provides benefits to more than 50 million Americans, at a critical moment.

The brazen killing of Thompson on Dec. 4 has ignited a broad conversation about the frustrations of navigating the U.S. healthcare system.

Many Americans, who routinely pay more for healthcare than people in any other country, have expressed anger over denied insurance claims, uncovered medical expenses, and requests for care being rejected.

The health insurance industry, on the other hand, is grappling with rising costs, driven by increased demand for healthcare under government-backed Medicare plans for older adults or those with disabilities.

Additionally, changes in Medicaid eligibility across states has left insurers with more patients who require more medical services.

UnitedHealth has reported higher-than-expected medical loss ratio - the percentage of premiums spent on medical care - a metric that tracks costs.

Noel, who joined in 2007, most recently served as the CEO of UnitedHealthcare's Medicare & Retirement unit.

The parent company also operates Optum, which includes a pharmacy benefits manager, specialty pharmacy, data analytics and doctor and surgical practices.

It also owns Change Healthcare, a technology company that was hit by a cyberattack last year, affecting thousands of healthcare providers and millions of customers.

(Reporting by Mariam Sunny and Sriparna Roy in Bengaluru; Editing by Sriraj Kalluvila)

((Mariam.ESunny@thomsonreuters.com;))