Ronan, a TV journalist and activist, remembered Thaddeus, who died age 27, in an Instagram post on Sunday (25Sep16), by uploading two childhood photographs of his late sibling.

One showed his brother standing with the aid of leg braces and crutches, the other is of the two brothers smiling while playing with a dinosaur toy.

"Rest in peace, brother. I never knew anyone stronger, or who overcame more," the 28-year-old captioned the side-by-side photos.

Mia adopted the then 12-year-old Thaddeus, who was paraplegic due to childhood polio, from an Indian orphanage in 1994, two years after her split from Ronan's father Woody Allen.

Thaddeus' body was found in his car in Connecticut last Wednesday (21Sep16), and he was subsequently pronounced dead at a local hospital.

Coroners have since said he sustained a self-inflicted gunshot wound and ruled the death to be a suicide.

The Carrie star, 71, has also spoken out about her family's heartbreak over news of her son's death, posting a statement on Twitter.

"We're devastated by the loss of Thaddeus, our beloved son and brother. He was a wonderful, courageous person who overcame so much hardship in his short life. We miss him," her statement read.

"Thank you for the outpouring of condolences and words of kindness. If you or someone you love needs help, or if you want to support a group doing life saving work, please visit: https://afsp.org/."