The king is seen as a unifying figure in the politically turbulent country
Thai police have arrested two people for allegedly spreading rumours about the health of King Bhumibol Adulyadej.
One of them was charged with spreading false information through a computer that undermined national security.
King Bhumibol, 81, was admitted to hospital in September with fever and fatigue. His health is a highly sensitive topic in Thailand.
Rumours about the king's health triggered a slump in Thai stock prices in October.
Thai officials said Teeranun Wipuchanin, a former stock trader, was detained at Bangkok airport on Sunday.
She was later charged with feeding false information through a computer system, which undermined Thailand's national security.
She faces up to five years in prison and a $3,000 (£1,824) fine.
Ms Wipuchanin said she had translated an article by a foreign news agency and posted it online to share information with stock traders and internet users.
"Everybody on that day wanted to know what caused the market to fall. The stock market had already dropped and we did the translation in the evening," she was quoted as saying by Reuters news agency.
The other suspect, Katha Pajariyapong, was arrested in Bangkok. He reportedly posted a message on the same topic on a website.
The king, the world's longest serving monarch, was taken to hospital on 19 September.
He made his first public appearance late last month: Thai TV showed him in a wheelchair touring the hospital grounds.
The Royal Household later said the king was in a stable condition and it would no longer issue daily updates on his condition.
King Bhumibol is seen as unifying figure in a country that has had repeated coups and coup attempts and 27 different prime ministers during his 60-year reign.