(Reuters) - UnitedHealth Group reported fourth-quarter revenue below Wall Street estimates on Thursday, hurt by weakness in its health insurance unit, sending the healthcare conglomerate's shares down about 3% in premarket trading.

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 also reported a higher annual medical cost ratio - the percentage of premiums spent on medical care - of 85.5%, compared with 83.2% in 2023. Analysts were expecting a ratio of 84.96%, according to data compiled by LSEG.

The health insurance industry has been grappling with increased costs for nearly two years due to a surge in demand for healthcare services under government-backed Medicare plans for older adults or individuals with disabilities.

Changes in enrollment norms for Medicaid plans have also left insurers with those who require more medical care, weighing on insurers' 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.

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