Deputy Prime Minister and Defence Minister Prawit Wongsuwon is in a hospital in Europe and is being treated for Meniere's disease, according to a source close to him.
Gen Prawit,72, was absent from the cabinet meeting on Tuesday, a day after the bomb explosion at the army's Phramongkulklao Hospital in Bangkok. He has been away since May 16.
The source said Gen Prawit had hearing problems and episodes of dizziness while walking.
Two weeks ago Gen Prawit had suffered ear pain. He wore a face mask and admitted only that he had a fever.
The source said Gen Prawit went to Europe with two military aides, but refused to name the hospital or the country were he was being treated. The treatment would take about another week.
Meniere's disease is an inner ear disorder. Patients may feel dizziness and suffer tinnitus, hearing loss and ear pain.
Gen Prawit phoned a reporter late on Wednesday and said he would be back at work on Thursday. A source said he was returning from Europe on Wednesday.