Rob Gronkowski endured nine surgeries and multiple injuries to nearly every part of his body during his NFL career.

And that includes his head.

In an interview Wednesday night with NBC News' Philip Mena, the retired New England Patriots tight delved into the impact that nearly two decades of football had on his brain.

Gronkowski admitted he had "mood swings" that he believes were related to brain injuries suffered on the field and told Mena he's currently doing brain exercises that include working on a 750-piece puzzle at his home.

The 30-year-old then shared a rather disturbing detail about how his head was physically affected from the repeated impacts of an NFL career.

"No lie, I felt my head, I used to have liquid," Gronkowski said. "It used to be thick, like, my head used to be thicker -- a centimeter of liquid in some spots. And you'd be like, what the heck? You could put indents in my head."

Gronkowski, who announced a partnership with a line of Cannabidiol-based pain relief products called CBDMedic on Tuesday, added he's now getting treatment and doing things to keep his brain "active."

The ex-tight end didn't share any further details, but it's possible he was dealing with some form of hydrocephalus, which literally translates to "water on the brain" and may result from head trauma.

Gronkowski suffered two confirmed Grade 1 concussions during his career (in 2013 and 2018) while enduring countless other minor hits, and admitted the state of his brain amid the current concerns surrounding CTE in NFL players played a role in his retirement.

"I was aware of what was going on with my body and my mind, and that's why I had to walk away," he added.

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