She said her family would not rest until she was returned.
US television host Savannah Guthrie has posted a message to her mother's kidnapper, asking them to provide proof that she is alive.
Authorities believe 84-year-old Nancy Guthrie was taken from her Tucson-area home in Arizona on Saturday night and are investigating what appears to be ransom notes sent to the media.
In a social media post today, Guthrie said her family is "ready to talk" to the kidnappers.
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https://www.instagram.com/p/DUW3mpqD8qd/"We, too, have heard the reports about a ransom letter in the media," the Today anchor said alongside her siblings.
"However, we live in a world where voices and images are easily manipulated.
"We need to know, without a doubt, that she is alive and that you have her.
"We want to hear from you and we are ready to listen. Please, reach out to us."
The three siblings pledged to do everything they could to get their mother back, who, Guthrie added, is fragile, lives in constant pain and needs medication to survive.
"Everyone is looking for you, mummy, everywhere," she said.
"We will not rest. Your children will not rest until we are together again.
"We love you, mum. Stay strong."
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At least three different media outlets, TMZ and CNN Tucson affiliates KOLD and KGUN, report receiving ransom notes earlier this week demanding millions of dollars in bitcoin for Nancy's safe return.
According to the outlets, the notes also described specific evidence in her home.
Authorities have not confirmed whether the notes are legitimate but are investigating.
"We are taking all tips and leads very seriously," the Pima County Sheriff's Department said.
"Anything that comes in, goes directly to our detectives who are co-ordinating with the FBI."
Authorities are also trying to piece together the timeline of Nancy's disappearance.
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Nancy had dinner with family members who took her back to her one-storey home, where she lives alone, in Tucson about 9.30pm on Saturday.
She is believed to have been taken just 15 minutes later.
Her pacemaker last sent a signal to her iPhone, which was left at the home, about 2am on Sunday.
Nancy was uncharacteristically absent from church later that same morning, causing a friend to call her family who discovered she was not at home.
Samples and evidence have been collected from the home but authorities have not identified any suspect or person of interest.
"Our mum is our heart and our home," Guthrie said.
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