The 86-year-old king, the world's longest-serving monarch who is treated as a near-deity in politically turbulent Thailand, left the Bangkok hospital on August 1 last year for his coastal palace in southern Hua Hin.
He lived in the hospital for almost four years after being admitted with respiratory problems in 2009 -- but there was no explanation from the palace over his prolonged stay.
"Now, it's time for a team of royal physicians to give him a thorough medical check-up which needs special equipment," a televised statement from the Royal Household Bureau said.
Bhumibol, who is officially King Rama IX, was accompanied to the Hua Hin palace by Queen Sirikit, who had also been hospitalised for a year.
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She will be 82 next week and was last seen in public as the royal couple were driven from Bangkok last summer to streets lined with adoring well-wishers.
Bhumibol, a keen saxophonist, photographer and sailor who is also a father figure to Thais, has made several public appearances since relocating to Hua Hin.
On July 22 he granted an audience to the leader of a military coup, Army Chief Prayut Chan-O-Cha, giving approval to an interim constitution mapping out year-long political reforms.
Born on December 5, 1927 in the United States, Bhumibol came to the throne aged 18 in 1946.
After finishing his studies in Switzerland he was crowned in 1950.
Thais see him as a unifying figure during a more than six-decade reign that has been flecked by political turmoil in the bitterly divided country.
The latest chapter of Thailand's political drama has lasted nearly a decade, with the kingdom roiled by political unrest -- and periodic bouts of violence -- since billionaire former premier Thaksin Shinawatra was toppled in an earlier coup in 2006.
Portraits of the monarch can be seen everywhere, while cinema-goers stand to attention when the royal anthem plays at the start of films.
Under the strict lese majeste rules anyone convicted of insulting the king, queen, heir or regent faces up to 15 years in prison on each count under strict royal defamation laws.