Currently, all Medicare cards have your Social Security number printed on them. So if you lose your Medicare card, your Social Security number may be compromised: possibly leading to identity theft.
Beginning in this month, Medicare will start mailing out new cards with randomly-generated Medicare Beneficiary Identifier numbers instead of your Social Security number. These cards will be mailed out to the address you have on file with the Social Security Administration in phases by geographic location.
Until you receive your new card, continue to use your current Medicare card. When you do receive your new card, be sure to shred your old one.