Private Practice stars Kate Walsh as Dr. Addison Forbes Montgomery. (ABC Handout)

Hit U.S. medical drama Private Practice is to end after its current sixth season.

Creator Shonda Rhimes announced the "heartbreaking" decision on Friday night, but insists she is "extremely proud" of the Grey's Anatomy spin-off.

In a statement, she writes, "I'm sad to say that Private Practice's run will end after... this season. There was a lot of discussion and debate but, in the end, the guys at the network and the studio and I all decided that Private Practice was reaching its finish line.

"Creatively, we are all extremely proud of the show and especially proud of this season - which you will all soon discover is a creative renaissance. I can't wait for you to see it.

"It's heartbreaking to end. Truly. I feel so lucky to write for these characters and tell these stories and I truly feel this show has taught me so much as a writer. And there's a family here, of crew and cast, people who have bonded over these past five and a half seasons.

"It's rare for a show to go for more than 100 episodes and we are incredibly proud to have been such a show. And we're grateful to all of you for spending time with us for an hour every week for the last six years. It's meant the world."

Actress Kate Walsh has been portraying lead character Dr. Addison Montgomery in the show since its premiere in 2007, but announced earlier this year she would be leaving the series after season six.