"Madoff was moved within the complex from the medium to the medical center in Butner on December 18," said Traci Billingsley, Chief Public Information Officer. "The potential reasons for an inmate's transfer are numerous and we don't release those specific reasons."
A lawyer for Madoff, tells the AP, his client has experienced dizziness and high blood pressure.
The Bureau of Prisons said Thursday that the 71-year-old disgraced financier remains under medical care for a seventh day.
Prison officials have declined to reveal why he was transferred from a North Carolina federal prison to a hospital. But bureau spokeswoman Traci Billingsley did say there have been no assaults at the prison in the past week.
Lawyer Ira Sorkin tells The Associated Press that Madoff has "had some dizzy spells and some high blood pressure."
Madoff has been imprisoned since March, when he pleaded guilty to fraud charges and admitted cheating thousands of investors out of billions of dollars.
The 71-year-old Madoff was arrested a year ago after he confessed to his sons and later to the FBI that his private investment business was a fraud.
He is being treated at the Federal Medical Center, part of the Butner Federal Correctional Complex in Butner.
Butner is located near the cities of Durham, Raleigh and Chapel Hill.
(The AP contributed to this report)