Letterman asked Hanks on Monday's edition of the program why he looks slimmer than usual, ABC News reported Tuesday.
Hanks, 57, told Letterman he has been dealing with high blood sugar for about 20 years, but his health recently took a turn for the worse.
"Well, you've graduated. You've got Type 2 diabetes, young man," Hanks quoted his physician as saying.
The "Captain Phillips" star assured Letterman the condition is controllable, but quipped, "Well, something's going to kill us all, Dave."