Of the six, only a boy aged 16 had showed signs of radiation exposure but was now in good health, a spokeswoman for Hidalgo state's health department said.
The people were taken to the general hospital in Pachuca for testing and after being cleared were turned over to authorities in connection with the gunpoint theft of the lorry outside Mexico City on Monday. The cobalt-60 it was carrying was from hospital radiotherapy equipment.
Hidalgo health minister Pedro Luis Noble said the men suffered from skin irritation and dizziness, but none was in a serious condition.
The agency said the volume of cobalt has an activity of 3000 curies: "It would probably be fatal to be close to this amount of unshielded radioactive material for a period in the range of a few minutes to an hour," it said.
The theft triggered alerts in six Mexican states and Mexico City, and notifications to the US and the International Atomic Energy Agency. It raised concerns the material could be used to make a dirty bomb, a conventional explosive that disseminates radioactive material.
Mexican authorities said the thieves seemed to have wanted the lorry, which has a moveable platform and crane, and probably did not know about the dangerous cargo.
The lorry driver stopped to rest at a petrol station when two armed men made him get out, tied his hands and feet, and left him nearby. The lorry was found abandoned on Wednesday 24 miles from where it was stolen, with the container for the radioactive material opened.
The cobalt-60 pellets were left about half a mile from the truck in a field, where authorities said they would have been a risk only to those who had handled them.
The pellets were from a hospital in the northern city of Tijuana and were being taken to a nuclear waste dump. Authorities maintained a 500-yard cordon around the site where the cobalt-60 still remains in the state of Mexico and were continuing to extract it safely, said Juan Eibenschutz, director general of Mexico's National Commission of Nuclear Safety and Safeguards.
"It's quite an operation and it is in the process of being planned," he said. "It's highly radioactive, so you can't just go over and pick it up. It's going to take a while to pick it up."