Thomas was diagnosed with pneumonia Sunday, after experiencing flagging energy in recent weeks, his office told the Capitol Hill newspaper The Hill. Thomas underwent tests at the Bethesda National Naval Medical Center this week -- where a bone marrow biopsy revealed leukemia.
Doctors recommended that he begin treatment immediately, Thomas' office said. Thomas is to remain at the center for treatment -- which could last four to six weeks -- to avoid the risk of infection, The Hill said.
Dr. Brian Monahan -- the director of Hematology and Medical Oncology at the center -- said most people in Thomas' "great physical condition and general health" are able to attain remission.
Thomas -- who was easily re-elected Tuesday to a third term -- said he expects to be back at work by the start of the next Congress in January.
Notable deaths of 2014 [PHOTOS]