Brett Favre Says Parkinson's Diagnosis Came In January After Struggles With Right Arm

Brett Favre tells TMZ Sports he was diagnosed with Parkinson's Disease in early January ... after he began experiencing trouble using his right arm and hand.

The Hall of Fame quarterback made the revelations to us in late August while discussing various football topics. At the time, he requested we don't report it, but, after he went public with his diagnosis during a hearing on Capitol Hill on Tuesday -- he gave us permission to run it.

Favre explained he first had suspicions something was up when his dominant arm -- which he used to throw 508 touchdown passes in his 20-year NFL career -- would get "stuck."

He said he'd push it down, forget about it ... but seconds later "it would be right back there."

He also revealed despite having no issues with strength -- when he'd go to use a screwdriver, he couldn't simply hold it steady with one hand.

But he said what ultimately led him to go to the doctor was his struggles to get his throwing arm through a jacket hole.

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

Favre says after he was formally diagnosed with the degenerative nervous system disorder -- five specialists told him they believed it was all likely due to the hits he suffered while calling signals in the NFL.

"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 -- who once said he might have suffered over 1,000 concussions in his career --- did appear optimistic about his prognosis ... telling us one of his docs said most people are far more ravaged by the disease by the time they learn they have it.

On Tuesday evening -- hours after the world learned of his ailment -- he issued a statement on his social media page expressing gratitude for all of those praying for him.