BANGKOK (REUTERS) - Thailand's King Bhumibol Adulyadej is in good overall health, the palace said on Monday, with doctors administering medicine to tackle stomach inflammation detected during a routine checkup.

The 86-year-old king was admitted on Wednesday to Bangkok's Siriraj hospital, where he was treated for nearly four years from 2009 to 2013 for a number of ailments. There were no details on the expected length of his current stay in hospital.

King Bhumibol, the world's longest-reigning monarch, has rarely been seen in public since leaving the hospital in 2013 and lives at the Klai Kangwon Palace - which translates as "Far from Worries Palace" - in the seaside town of Hua Hin.

According to a statement by the Office of His Majesty's Principal Private Secretary, the king's respiratory system, lungs, body temperature and blood pressure were normal.

Doctors had advised him to take dietary supplements fed directly into his bloodstream to boost nutrient intake and build strength.

Queen Sirikit, who turns 82 on Tuesday, is also undergoing a health checkup at the same hospital.

The king is a constitutional monarch who is seen as above the political fray.

Despite several interventions in the 1970s and 1990s to call for calm during Thai political crises, he has stayed silent during power struggles in recent years.

The intractable upheaval since 2005 broadly pits supporters and allies of former populist Prime Minister Thaksin Shinawatra against a military-backed conservative elite that despises him and accuse him of disrespect to the revered monarchy. Thaksin vehemently denies that.