Two shots behind Lee entering the final day of the two-week race on the San Francisco Peninsula Tour, Castren birdied the first three holes, tied the fifth par 5 and birdied the ninth par. 5 for a 30 before nine.
Castren followed with eight straight pars and birdied 3 feet on the 18th par-5 for a 7-under-65. the women’s US Open at the nearby Olympic club.
Lee finished with a 69. She reduced Castren’s lead to one with a birdie on the 15th par-5, but lost a stroke with a two-putt bogey on the 17th par-3.
Lee missed a chance to become the first player to follow a Symetra Tour win with an LPGA Tour win on her next start. The 26-year-old from Taiwan won the Mission Inn Resort and Club Championship two weeks ago in Florida on the development tour.
Full field scores of the Mediheal LPGA Championship
Hannah Green (66) and So Yeon Ryu (67) are tied for third at 8 under.
Lexi Thompson shot a 74 to tie it 34th to 1 more. A week ago at the Olympics, she lost a five-stroke lead in the final round, playing the last seven holes in 5 to complete a stroke in a playoff won by Yuka Saso. Women’s Open playoff loser Saso and Nasa Hataoka skipped the event at Lake Merced.
Michelle Wie West finished with a 71 tied for tied 40th after missing the weekend cups in her first four events of the year.