Police are blocking access to the hospital, in central Hidalgo state.
The truck carrying medical equipment with radioactive substances inside was stolen in the area earlier this week.
A container housing the radioactive waste was found abandoned two days later close to where the truck had been stolen, the authorities said.
Anyone who came into contact with the material – cobalt-60 – faced life-threatening levels of radiation, officials warned.
Hospitals in the area had been told to look out for people with symptoms of radiation poisoning.
Police moved in and cordoned off the General Hospital of Pachuca, capital of Hidalgo state, when six people were admitted complaining of nausea, dizziness and other possible signs.
