He did not mention Thailand's ongoing political crisis, which was sparked by a 2006 coup that ousted Thaksin Shinawatra, the prime minister.
The Royal Household Bureau says King Bhumibol, the world's longest-reigning monarch, is recovering from lung inflammation and is in no danger.
The king has appeared in public only twice since he was hospitalised on September 19.
People outside the hospital, many of them wearing pink as astrologers have said it is an auspicious colour for the king's health, chanted "Long live the king" as he appeared.
Thanom Chumpoo, a Bangkok resident, said she had been visiting the hospital every day, hoping the king would make an appearance.
"I am glad that he is recovered. I am so overwhelmed to see him and want him to live forever to guard us," she was quoted by the AFP news agency as saying.
King Bhumibol is widely revered as the father of the nation, and many Thais regard him as almost divine.
His birthday is a huge event, marked by a public holiday and celebrated across the country with fireworks and Buddhist rituals.
But this year, many events, including the monarch's customary birthday speech, have been cancelled.
Crown Prince Maha Vajiralongkorn, King Bhumibol's son, was due to represent his father in ceremonies on Saturday evening and Sunday.
King Bhumibol was also unable to deliver his birthday address last year because of ill health.