Kate Upton flashes her behind in sheer dress at birthday bash

Kate Upton didn't quite step out in her birthday suit -- but she came close.

The supermodel had an early birthday celebration at The Blond at 11 Howard Hotel in New York City on Wednesday night, ahead of her turning 24 on Friday. Upton bared her butt in a thong bodysuit that she wore under a sheer, star-embellished dress. While posing with her fiance, 33-year-old MLB pro Justin Verlander, the Sports Illustrated Swimsuit cover girl got cheeky with photographers and flashed her bum.