Immediately after the crown prince's short address, his sister, Princess Sirindorn, spoke to the nation to say the king was suffering from bronchitis and inflammation of the esophagus.
Sirindhorn added that his condition was "not serious", saying he had "a blockage in his throat and a poor appetite".
Thais had been looking forward to hearing from their monarch, who has long been looked to for guidance in times of political turmoil.
Thailand is in the middle of a long-running political stand-off.
Airport officials in Thailand are rushing to resume operations at Bangkok's two main airports after anti-government protesters lifted their week-long siege.
The king's speech is usually given in front of senior government officials on the evening before his birthday and broadcast live on national radio to the nation.
Return to normalcy
The main Suvarnabhumi airport began limited international operations on Thursday with staff saying they hoped to resume near normal service by Friday.
|Bhumibol's condition is described as 'not serious' [AFP]
The siege, which ended after a court ruling forced the governnment to stand down, had stranded an estimated 300,000 travellers and cost the Thai economy losses estimated in the hundreds of millions of dollars.
But while the crisis that led to the airport occupation may have eased with the peaceful overthrow of the prime minister, Somchai Wongsawat, political tensions remain high with many issues still unresolved.
"It is nothing more than an intermission," Charnvit Kasetsiri, a historian and former rector of Bangkok's Thammasat University, told the Associated Press news agency.
"It is not over until the two sides of the political spectrum can reconcile and the prospect of that happening is very bleak."
On Wednesday protesters from the People's Alliance for Democracy (PAD) began clearing out of Bangkok's Suvarnabhumi and Don Muang airports, a day after Thailand's constitutional court forced Wongsawat out of office.
Guilty of fraud
The ruling banned him from politics for five years and dissolved his People Power Party after finding the party and its leaders guilty of election fraud.
Nonetheless PAD leaders have warned they will resume their protests if the new government continues to have links to Thaksin Shinawatra, the former prime minister ousted in a 2006 coup.
Somchai is Thaksin's brother-in-law.
Although living in exile and recently convicted in absentia of corruption, Thaksin remains massively popular among Thailand's rural poor and the new government is almost certain to include his allies.
With the PAD emboldened by their latest victory, that could set the stage for further confrontation.