The heart rate monitor on the device helped save the life of an 18-year-old suffering from undiagnosed kidney disease.

According to WFTS, 18-year-old Deanna Recktenwald, of the Tampa area, was at an area church recently when her Apple Watch gave her a notification: her resting heart rate had reached 190 beats per minute, recommending that she seek medical attention.

Her mother, a registered nurse, then took her to a walk-in clinic, and later to an emergency room, where doctors gave her a diagnosis of chronic kidney disease, for which she had expressed no previous symptoms.

Stacey Recktenwald, Deanna's mother, later wrote a letter to Apple.

"If it wasn't for her Apple watch alarming her about her HR we wouldn't have discovered her kidney issue. I honestly feel your Apple Watch saved my daughter's life," Stacey wrote. "I am forever grateful to Apple for developing such an amazing, lifesaving product."

Tim Cook reportedly wrote back personally, thanking the Recktenwalds for sharing their story.