Thursday night’s shocker — well to some anyway — on ABC’s “Grey’s Anatomy” helped boost the veteran medical drama to its largest audience since its September season premiere.

Thursday’s episode saw the exit of one of the original castmembers, a plot point that had been leaked earlier Thursday in an Entertainment Weekly report and then confirmed by ABC late last night. The magazine hits newsstands on Friday, but most subscribers get it on Thursday, and many “Grey’s” fans knew what was coming in last night’s episode.

That certainly helped pump up the ratings for “Grey’s” on the opening night of the May sweep. According to preliminary national estimates from Nielsen, “Grey’s Anatomy” averaged a 2.8 rating/10 share in adults 18-49 and 9.4 million viewers overall in the 8 o’clock hour — matching the show’s best demo score since its season premiere in September and also drawing its largest overall audience since then.

It was up 33% in the demo from the previous week (2.1) and 22% higher than the 2.3 it did on the comparable evening in the 9 o’clock hour a year ago.

“Grey’s Anatomy” was the night’s No. 2 program in adults 18-49, behind only CBS’ “The Big Bang Theory” (3.5 rating/13 share), which was also the most-watched Thursday program overall (14.4 million viewers). “Big Bang” was down a bit from last week’s preliminary 3.7, which went up to a 3.9 in the nationals.

Elsewhere, the “Grey’s” gains did nothing to help “Scandal” at 9 (2.3/7 in 18-49, 7.8 million viewers overall), with the drama down a tick from last week but still strong enough to rule its hour in key demos; it was the night’s No. 2 drama in 18-49. Also down a tenth from last week was “American Crime” at 10 (0.9/3 in 18-49, 4.1 million viewers overall). Next week, “Scandal” will be preempted to make for a two-hour episode of “Grey’s Anatomy” that will deal with the aftermath of last night’s episode.

ABC tied CBS for the night’s lead in adults 18-49, which would mark the first time since 2009 that the former has ranked No. 1 on the opening night of the May sweep.

At CBS, the lower “Big Bang Theory” led into a slightly softer result at 8:30 p.m. for “The Odd Couple” (2.0/7 in 18-49, 9.5 million viewers overall), though it held up well considering the strong showing in the 8:30 p.m. half-hour by ABC’s “Grey’s Anatomy.” In its new 9 p.m. time period, “Mom” (1.9/6 in 18-49, 9.0 million viewers overall) came back to earth a little after its strong 2.2 rating last week when NBC’s “The Blacklist” was in repeat; it placed second to “Scandal” in 18-49 and edged out the ABC drama in 25-54 (2.9 to 2.8). Also lower were the 9:30 p.m. repeat of “Big Bang” (1.7/5 in 18-49, 8.3 million viewers overall) and 10 p.m. drama “Elementary” (1.3/4 in 18-49, 7.6 million viewers overall).

At NBC, a repeat of “The Blacklist” at 8 p.m. (0.5/2 in 18-49, 3.4 million viewers overall) was followed an original at 9 (1.4/5 in 18-49, 7.2 million viewers), which was down 0.2 (or 0.1 depending on round-offs in the nationals) from its most recent original. While the James Spader drama has seen its number decline on Thursdays this season, it’s worth remembering how much lift it gets in DVR playback; last week’s episode grew by 84% (or 1.34 ratings points) in just three days (2.03 vs. 1.59). Closing the night for NBC, “Dateline” (0.9/3 in 18-49, 5.8 million viewers overall), drew the timeslot’s largest overall audience since “Peter Pan Live” in December.

Fox’s “Bones” (1.0/3 in 18-49, 4.6 million viewers overall) and “Backstrom” (0.8/3 in 18-49, 3.0 million viewers overall) held steady week to week, as did CW’s “The Vampire Diaries” (0.6/2 in 18-49, 1.4 million viewers overall) and “Reign” (0.3/1 in 18-49, 0.8 million viewers overall).

Telemundo continues to score well at 10 p.m. with the third season of “El Senor de los Cielos” (Lord of the Skies), which did a 1.2/4 in 18-49 and 2.3 million viewers overall in the preliminaries. It ranked No. 1 among adults 18-49 in the nation’s top three markets (New York, Los Angeles and Chicago).

Preliminary 18-49 averages for the night: ABC and CBS, 2.0/7; Univision, 1.0/3; NBC and Fox, 0.9/3; Telemundo, 0.7/3; CW, 0.5/2.

In total viewers: CBS, 9.4 million; ABC, 7.1 million; NBC, 5.5 million; Fox, 3.8 million; Univision, 2.6 million; Telemundo, 1.7 million; CW, 1.1 million.