The health check came a day after she lost a release bid and broke down in a Malaysian court, her lawyer said.
Doan Thi Huong is on trial for the 2017 Kuala Lumpur airport assassination of Kim Jong-nam. Her Indonesian co-accused was released this week after prosecutors dropped a murder charge, but authorities Thursday refused to do the same for Huong.
The 30-year-old was brought from prison to a Kuala Lumpur hospital in a van accompanied by several police cars and heavily armed officers wearing balaclavas, according to one of the officers escorting her. She left after about an hour.
"Our client Doan is stressed, she has been taken to a public hospital. She is undergoing a psychiatric assessment," her lawyer Hisyam Poh Teh Peik told AFP.
"Let us all pray for her health and immediate freedom."
On Thursday the suspect sobbed in the dock and had to be helped out of court by two police officers after a prosecutor announced the attorney-general had refused to drop a murder charge against her.
It was a shock development after her Indonesian co-defendant, Siti Aisyah, was freed on Monday and flew back home.
Authorities did not give any reason for withdrawing the charge against Aisyah, which followed an intense lobbying effort by Jakarta.
Both women have always denied murder, insisting they were tricked by North Korean spies into assassinating Kim by smearing a nerve agent on his face, and believed it was a prank for a reality TV show.
Vietnam's foreign ministry expressed "regret" at Thursday's outcome while one Malaysian lawmaker criticized it as "mind-boggling."
Huong is now the only person on trial for the Cold War-style killing, and could face the death penalty if convicted. Her lawyers say she is a scapegoat and the masterminds were four North Koreans who fled Malaysia shortly after the assassination.
South Korea accuses Pyongyang of plotting the murder of Kim Jong-un's estranged relative, who was once seen as heir apparent to the North's leadership. The North denies the claim.