MSNBC on Tuesday announced its coverage plans for the highly anticipated testimony of former FBI Director James Comey to the Senate Intelligence Committee on Thursday, but its biggest star, Rachel Maddow, is noticeably absent from the network's on-air roster.

"MSNBC’s coverage of the hearing starts at 9 a.m. ET / 6 a.m. PT with Brian Williams joined by NBC News’ Nicolle Wallace and a slate of expert guests beginning at 9:30 a.m. ET / 6:30 a.m. PT," reads a release from the network.

Williams has co-anchored all big political events with Maddow on MSNBC for more than a year, including on election night.

Maddow, 44, has not hosted her highly rated program since May 25, with MSNBC chief legal correspondent Ari Melber and "AM Joy" weekend host Joy Reid filling in.

Maddow tweeted Tuesday that she will be back on the air Tuesday night, in just her second tweet since May 26.

“I'll be back on the air tonight -- 9pm ET MSNBC. Huge heartfelt thanks to everyone who sent good wishes while I was out -- happy to be back!” she wrote.

She did call in to a Melber-hosted show on June 2 to discuss recent developments of a special counsel investigation into Russian ties with the Trump campaign, her only on-air appearance since May 26.

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The former "Air America" personality has covered Russia more than any other cable news host, with an analysis by the online publication The Intercept showing Maddow covering the investigation of ties between Russia and the Trump campaign more than every other issue and news item combined.

Daily Kos blogger LibbyD expressed her concern for Maddow's health last week on the popular progressive site.

“I am really getting concerned and I’m not alone. She has been ‘under the weather’ for almost a week now and she hasn’t tweeted either during this time,” she writes.“Now I am not quick to jump to conclusions but this thought crossed my mind before she was even sick.

“To think she is on the Kremlin’s radar is not absurd," she continues. "Her journalism is remarkable and I think she’s getting closer and closer to the truth. We all know what Putin does to his critics and opposition leaders. Will he go as far to do something to Rachel?”

The host's status has caused concern on social media.

OK, i gotta ask, Has @maddow been poisoned by some #Russians?#Maddow you don't get to stay sick!

We love your replacements, but c'mon! — EvaDestruction (@isis_whitehead) June 1, 2017

Where the heck is Rachel?? @maddow She is never sick this long. Seriously #terrified right now. — Chloe Randolph-Brown (@ChloeTheGr8st) June 6, 2017

What the hell is a actually wrong with Rachel? She's been sick since May 26th. #maddow — Oregonemom (@Oregonemom) June 6, 2017

Maddow had the top-rated program in cable news in the key 25-54 demographic during the month of May.

The Hill has reached out to Maddow's spokeswoman at MSNBC for comment.

- Updated at 11:26 a.m.