Jan 16 (Reuters) - UnitedHealth's fourth-quarter revenue missed Wall Street estimates on weakness in its health insurance business and the company also reported higher-than-expected annual medical costs, sending its shares down nearly 5% before the bell on Thursday.

The company's results come a month after Brian Thompson, the CEO of UnitedHealth's insurance unit was killed, igniting a conversation over frustrations related to navigating the U.S. health insurance system.

UnitedHealth's annual medical cost ratio — the percentage of premiums spent on medical care — rose to 85.5%, compared with 83.2% in 2023. Analysts were expecting a ratio of 84.96%, according to data compiled by LSEG.

Companies typically aim for a ratio close to around 80%, but the industry has been grappling with increased costs for nearly two years due to high demand for healthcare services under government-backed Medicare plans for older adults.

Shares of insurers CVS Health and Elevance Health also fell more than 3% in premarket trading.

Industry bellwether UnitedHealth has encountered a series of challenges throughout 2024, including a cyberattack on its tech division and a persistent increase in medical costs.

UnitedHealth reported revenue from premiums of $76.48 billion for the fourth quarter ended Dec. 31, compared with estimates of $78.06 billion.

Overall revenue for the quarter came in at $100.81 billion, below expectations of $101.76 billion.

Revenue at its Optum healthcare services unit, which includes the pharmacy benefit business OptumRx, rose 9% to $65.1 billion.

On an adjusted basis, UnitedHealth earned $6.81 per share in the fourth quarter, compared with estimates of $6.72 per share.

The company reaffirmed its 2025 adjusted profit forecast of $29.50 to $30.00 per share, that it had initially provided in December.