The 87-year-old king, who is perceived as a near-deity by many Thais, has been in Bangkok's Siriraj Hospital since being re-admitted in May, but information on his condition has been scarce.
Doctors at the hospital have "reduced the level of water in his brain", the Royal Household Bureau said in a statement on Monday.
"Water on the brain" is an outdated phrase that more correctly refers to a condition known as hydrocephalus. The "water" is actually cerebrospinal fluid (CSF) - a clear fluid surrounding the brain and spinal cord.
Hydrocephalus in the elderly generally causes thinking and reasoning problems, difficulty walking and loss of bladder control.
"During his stay at the Siriraj, he has had fever periodically and a raised heartbeat - assumed to be caused from an earlier lung infection," the statement said, adding his heart rate had returned to normal after treatment.
In October 2014 the king was admitted to Bangkok hospital with fever and an increased heart rate. Blood tests showed signs of infection and a CAT scan revealed that the king's gallbladder was inflamed and swollen. The gallbladder was removed during a 75-minute laparoscopic operation and treated with intravenous antibiotics.
Some clues as to the monarch's health have been revealed via the Wikileaks publications. In November 2009, U.S. ambassador Eric G. John wrote that Bhumibol was:
"by many accounts beset long-term by Parkinson’s, depression, and chronic lower back pain…"
There have been rumours that he has been receiving experimental stem cell therapy.
The Australian.com also reported that Thai Deputy Prime Minister Suthep Thaugsuban is quoted as telling a US diplomat that the problem was the king's mental health. "Suthep confirmed . . . that King Bhumibol exhibits classic symptoms of depression (and) that the really (sic) worry was his state of mind, depressed at the state of affairs in his kingdom at the end of his life".
The Thai king's health is a controversial topic and reporters generally do not speculate about it for fear of prosecution. In February 2015 a 25-year-old man was detained for suspected royal defamation after he allegedly posted a forged report on the health of the country's monarch online.
The statement, which was recreated to look like an official palace notice, quickly spread via social media and messaging apps.
King Bhumibol is regarded as a unifying father figure in a nation beset with bitter political divisions, and there are profound concerns over the kingdom's future, an country's stability, as his reign enters its twilight years.
Any unofficial commentary on the king's health is regarded as one that incites unrest.
Fears over Thailand's future among competing elites as Bhumibol's reign enters its twilight years are seen as a motivating factor behind a decade of political turmoil in the kingdom.
Since 2006, the nation has witnessed two coups, the removal of three prime ministers by the courts and several rounds of street protests that have often ended in violence.
Royalist generals seized power in a coup in May last year after weeks of protests against the civilian government of Yingluck Shinawatra, whose family and their proxies have won every election since 2001.
The Shinawatras are loathed by the royalist elite who accuse them of widespread corruption and subverting the kingdom's political status quo.
Thailand's generals have said they will hand back power once the constitution has been rewritten and corruption expunged.
But critics say the military has used its self-designated status as the defender of the monarchy as a pretext to grab power and ensure the Shinawatras never return to politics.
The king's health is a controversial topic. The Thai monarchy is shielded by one of the world's toughest lese majeste laws and prosecutions have increased dramatically since the military took over.
A man was jailed for 30 years on in 7 August 2015 for "insulting" the monarchy on Facebook, in one of the toughest known sentences for royal defamation. The same day a woman received a 28-year jail term for the offence.
Media have to routinely self-censor when reporting on the monarchy for fear of falling foul of the broadly worded law, which carries up to 15 years in jail for each count of insulting the king, queen, heir or regent.
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