The 84-year-old had cancelled engagements from Monday after suffering dizziness and nausea caused by the condition, known as cerebral anaemia, AFP reports.
A palace spokesman said the monarch on Thursday was “doing his official duties as usual,” declining to comment further.
On Wednesday the palace had reported Akihito’s health was improving.
The monarch has previously had surgery for prostate cancer and heart problems, and the state of his health is carefully watched in Japan.
He shocked the country in 2016 when he signalled his desire to step down after nearly three decades in the job, citing his age and health problems.
On Monday, a government spokesman said the monarch had been treated for cerebral anaemia, and the following day the palace said he was still suffering vertigo and a stomachache.
Akihito is set to be the first emperor to retire — on 30 April 2019 — in more than two centuries in the world’s oldest imperial family.
He will be succeeded by his eldest son.
The status of the emperor is sensitive in Japan — a legacy of the massive Pacific war waged in the name of Akihito’s father Hirohito, who died in 1989.
Akihito has embraced the more modern role as a symbol of the state, which was imposed on the royal family after World War II ended. Previous emperors including Hirohito had been treated as semi-divine.