Mandy Moore mourns sudden death of her dog: 'My heart is utterly shattered'

Actress Mandy Moore is mourning the unexpected death of her dog, Joni.

The "This Is Us" star explained in an Instagram post that the 13-year-old rescue recently underwent a surgical procedure to remove a mass on her liver, but had been recovering "really well."

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However, on Monday night, Joni was unable to finish her dinner and couldn't seem to get comfortable, prompting Moore to bring her to the veterinarian's office.

"My heart is utterly shattered. She was my first love and best friend. Through every twist and turn of life of this past decade and change, she was right there," Moore wrote. "There’s a deep chasm of emptiness that will never be filled but I am so grateful for her sweet nature and the endless connection, compassion, responsibility, patience and devotion that the love of a pet can bring into your orbit."

Moore, 36, adopted Joni in 2008 and was "instantly smitten." Her husband of two years, Dawes musician Taylor Goldsmith, also reflected on the loss in an Instagram post, and added of Moore: "You've never seen a better dog mom."

"We’re gonna miss [Joni] so much. I referred to her as 'My Boss' - impervious to any of my efforts to get her to do anything she didn’t wanna do, always at my side when cooking or eating giving me the absolute sweetest look of love mixed with a little entitlement and expectation," he wrote."Never met a happier, kinder, more trusting (or better fed) dog in my life. And it’s 100% due to the life and love @mandymooremm gave to her. We miss you Joni!!!!"

Moore and Goldsmith are expecting their first child, a baby boy, next year.