Thaddeus passed away at Danbury Hospital in Connecticut on Wednesday (21.09.16) after a car crash but no foul play was thought to be involved.
According to gossip website TMZ, Connecticut police responded to a call at Roxbury, Connecticut, at around 12:45pm on Wednesday, and found Thaddeus "gravely injured in his car along Route 67".
The the 71-year-old actress' son was taken to hospital, and later pronounced dead at 2:30pm.
Thaddeus was one of 10 children adopted by the former fashion model, and was paraplegic - meaning he had loss of feeling in his legs - due to polio, a rare viral infection.
After she adopted Thaddeus, Mia spearheaded a campaign to rid the world of polio, revealing that she too had suffered from the infection as a child but was able to fight it due to living in a country with advanced healthcare.
She previously said: "I perhaps am more motivated than most people because I had polio myself and it was a real struggle to come through it, and what I saw will never leave me.
"Perhaps even more so because I have a son who is only 12 years old and who is paralysed from the waist down because of polio.
"This is a terrible disease, and he has a lot of difficulty just getting through the day. I would love to see the end of polio, where no more children have this."
In total, the 'Rosemary's Baby' actress had 14 children - Matthew and Sascha, 46, Soon-Yi, 45, Fletcher, 42, Summer, 41, Moses, 38, Dylan, 31, Ronan, 28, Frankie-Minh, 25, Isaiah, 24, and Kaeli-Sha.
Daughters Lark Song, 35, and Tam, 21, have also died, in 2008 and 2000 respectively.