The 36-year-old tight end -- who retired following the 2020 season -- revealed the situation Monday on social media ... saying, "This past week has been exceptionally challenging for our family."

"As many of you know, our son TJ has faced serious heart issues since birth. TJ has already undergone 3 open heart surgeries and has survived with a modified heart for his first 8 years of life."

Greg and his wife, Kara, have publicly talked about TJ's condition for years -- explaining he was born with a congenital heart disease.

Olsen continued, "Unfortunately, it seems his heart is reaching its end. We are currently working through the process to determine our next steps, which ultimately could lead to a heart transplant."

Olsen says TJ is currently being treated at Levine Children’s Hospital in North Carolina -- and praised the team for the "world class care" TJ is receiving.

FYI, Greg and Kara donated $2.5 million to the Levine Children's Hospital in 2019 to help fund a pediatric cardiovascular and congenital heart outpatient clinic.

"We don’t know how long we will be within these hospital walls," Olsen added ... "We do know that we are in full control of our attitudes and our outlook."

"TJ has been a fighter since birth. We are going to get through this as a family and be better off as a result of this experience."

The Olsen Family is getting a ton of support on social media from athletes, fans, reporters and even politicians.

Erin Andrews posted, "Thinking of you guys!!"

North Carolina senator Thom Tillis added, "Susan and I are praying for your family."