Authorities said Caroline Karimi was last seen by her family at her residence in the 2900 block of Queensbury Street around 7 a.m.
The 18-year-old also failed to show up to volunteer at San Gabriel Valley Medical Center which her family says is very unusual.
She is booked on a 7 a.m. flight to Boston Wednesday to attend a family gathering.
Karimi was described as white, standing 5 feet 5 inches tall, 135 pounds, with brown hair, green eyes, and fair complexion. She was last seen wearing a dark-blue hospital shirt, white pants and orange-blue shoes.
Karimi drives a 2007 green Toyota Prius with the California license plate 5YVV475.
Anyone with information about Karimi's whereabouts was urged to contact LAPD missing persons Detective Alma Mercado at (213) 996-1800.
CNS contributed to this report.
