Brett Favre Thanks Fans For Support After Parkinson's Diagnosis

Brett Favre is speaking out after his shocking announcement this week ... with the NFL legend saying he greatly appreciates the "unbelievable show of support."

The Green Bay Packers great posted video to his Instagram about 48 hours after testifying at a welfare reform hearing on Capitol Hill ... where he announced he has Parkinson's .

Wow, what an overwhelming amount of support from you all," Favre captioned the post, adding, "Thank you and this is just the beginning of another mountain to climb."

Brett also said he hopes his admission sheds light on head trauma and Parkinson's disease.

"There's a lot of people that are out there with it," he said. "Some know it, some don't know it. It can happen to anyone at any time."

While Favre went public with his illness at the hearing ... he told TMZ Sports back in August that he was diagnosed with the disease in early January.

Brett told us he met with five specialists ... and all believed the disease was likely a result of his long football career.

"They all said the same thing," he said. "'If it's not in your family' -- and there's none on either side of my family -- 'then the first thing we look at is head trauma.' Well, hell, I wrote the book on head trauma."

Favre told us he first started to believe something was wrong when his arm would get "stuck" while trying to put on a shirt.

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He also experienced issues while attempting to fix things around his home, even having trouble using a basic tool.

"I felt my arm," Brett said ... "the strength was there, but I could not guide it. And it was the most frustrating thing."