Lopez also had expressed interest in spending more time with his family. He and his wife, Renee, have a son and a daughter.
MLB Network insider Ken Rosenthal was the first to report Lopez's retirement, which was confirmed by sources close to the pitcher.
Lopez lent respectability to the term "LOOGY," an acronym for "left-handed one-out guy." Typically summoned to face formidable left-handed batters, Lopez thrived by relying on a low release point, deception and late movement on his pitches. Left-handed batters mustered a .202 average against him during his career.
Lopez particularly excelled after joining the Giants in a 2010 Trade Deadline deal. He recorded a 2.47 ERA during his tenure with San Francisco, compared with 4.37 for the other four teams that employed him during a Major League career that spanned 14 years. Lopez pitched for four World Series winners, bolstering San Francisco's championship bullpens in 2010, 2012 and 2014 after participating with Boston's victorious 2007 squad.
Right-hander Ryan Vogelsong heaped praise upon his former teammate.
"He was always a team guy," said Vogelsong, who recently signed with Minnesota. "Everything about our team and his ability to help everybody else and try to make everyone else better seemed like it trumped what he was doing. Everybody wants to do their thing and be good -- I know he did, too -- but you would have never known by the way he interacted with and rooted for his teammates and tried to make people better.
"The reason the teams won that he was on was because that's what they were all about. And I think he's probably one of the best factors of why those teams took on those characteristics."
A graduate of the University of Virginia with a degree in psychology, Lopez logged 839 appearances, tied with Arthur Rhodes for 11th all-time among left-handed relievers.
He averaged 68 appearances per year in his six Giants seasons, receiving frequent calls from manager Bruce Bochy, and the results explain Bochy's trust. Here's a sampling of how top left-handed batters fared against Lopez:
Carlos Gonzalez, .160 (4-for-25)
Chase Utley, .043 (1-for-23)
Bobby Abreu, .095 (2-for-21)
Jay Bruce, .100 (2-for-20)
Adrian Gonzalez, .211 (4-for-19)
Ryan Howard, .167 (3-for-18)
Joey Votto, .154 (2-for-13)
Robinson Cano, .100 (1-for-10)
Todd Helton, .100 (1-for-10)
Said Vogelsong: "Every time I turned around, he was facing Adrian Gonzalez or Carlos Gonzalez -- the best left-handed hitter in [the opponent's] lineup when we're winning the ballgame, sometimes three nights in a row. That's his only guy. Some people say, well, that's the easiest job in the world, because he had to get only one guy out. But nine times out of 10, the guy he had to get out was the best left-handed hitter in their lineup and the game was on the line. So it wasn't the easiest job. The success-to-failure rate was probably the slimmest and hardest one of any job you could have of any pitcher in the big leagues. And he nailed it for a long, long time."