The palace issued a statement saying the king had a high temperature and slight cough early on Sunday.
An X-ray showed a mild inflammation in his lower lungs, and he was given medicine and fluid intravenously.
The statement said his doctors found his temperature, blood pressure and breathing rate to be normal by afternoon.
Bhumibol entered Bangkok's Siriraj Hospital in 2009 with a lung inflammation and has stayed since then except for infrequent short trips.
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