Thai police have charged a fourth person with spreading false rumours about the health of the country's revered king, after concerns over his condition sent stocks plunging, police said Thursday.

Radiologist Tasaporn Rattawongsa, 42, was arrested at the private hospital in Bangkok where she works late Wednesday for damaging national security by posting false information online.

"We have formally charged her with committing computer crime by posting this information on the Internet," said Major General Kriengsak Arunsrisophon, head of the technology crime division.

Three other Internet users were arrested in early November on the same charge. All face up to five years' imprisonment and a fine of up to 100,000 baht (S$4170) if found guilty.

King Bhumibol Adulyadej, who is regarded as a demi-god by many Thais and is a stabilising force in a politically turbulent nation, has been in hospital for two months with a lung infection and fever.

The Thai bourse plummeted 8.22 percent at one point on October 15 over rumours about the health of the 81-year-old, the world's longest-reigning monarch, prompting the government to launch an investigation.

Police said Tasaporn had confessed to posting a message but said she did not intend to harm the royal family.

They said police had confiscated the woman's notebook from her Bangkok apartment for further investigation.

The three other suspects - Katha Pajariyapong, Theeranan Vipuchanin and Somjet Sitthiworakul - have all been released on bail.

Media rights group Reporters Without Borders (RSF) has called the charges "baseless" and said the defendants were being made "scapegoats" for the market falls.

The king was admitted to Bangkok's Siriraj hospital on September 19.

He made his second public appearance since being admitted to hospital on November 3, lighting candles at a traditional water festival. He was pushed in a wheelchair to the ceremony by doctors.

Thailand also has a strict lese majeste law, under which insulting or defaming any member of the royal family is punishable by up to 15 years in jail.