The resident was one of four tested for MERS. The other three, who have just returned from a trip to China, have already tested negative.
A thorough lab test on the fourth patient is expected later Friday, said a health official in Chiang Mai who insisted that there was no reason to be panic. He said measures are in place to handle the situation.
Prime Minister Gen Prayut Chan-o-cha said that 69 hospitals in Thailand specialised in SARS and bird flu are now assigned to be watchful for MERS cases.
Asked if he was confident in keeping the situation under control, he shot back at reporters. "Can I give order to the disease? It's up to all Thais who have to be on alert and contact the authorities if something suspicious happens. I can't give order to the disease."