The statement issued late Sunday raised concern about the frail health of King Bhumibol Adulyadej, who has spent most of the past six years living in a Bangkok hospital.
The Royal Household Bureau statement said the king had battled a fever for the past two weeks and that tests found "an infection in the lower part of the lungs" and an infection in his blood as well as inflammation in his right knee joint.
The statement did not make clear if the lung infection was gone.
Bhumibol, who took the throne in 1946, has faded from public life in recent years due to ill health.
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The deeply revered king is a constitutional monarch with no formal political role but has been regarded as a stabilizing figure for Thailand, which has experienced political turmoil in recent years.
Many regard him as a father figure and his December 5 birthday is celebrated as Father's Day in Thailand.