The palace issued a statement saying the king had a high temperature and slight cough early Sunday. An X-ray showed 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.
Thailand's monarchy once had near-universal respect among Thais, but the king's fading from public life and the palace's perceived role in the nation's political battles have tarnished the institution in recent years.
The monarch's health is closely followed by all Thais.