Hey Arnold! ran for almost eight years without ever clarifying one key point: we never got to actually meet Arnold's folks. Which makes this crazy Hey Arnold! theory about Arnold's parents, in which we're led to believe that Arnold's grandparents are actually his biological parents and that they've lied to him, all the more unsettling. The theory cites mental illness as the reason why Phil and Gertie would lie to their real son and even includes a medical explanation for why Arnold looks the way he does. Basically, it's going to shake any fan, no matter how casual, to their core and raise even more questions about the Shortman family set up.
According to show creator Craig Bartlett, "The story of Arnold’s missing parents just kind of grew: when I first pitched Hey Arnold! to Nickelodeon, I was really glib about them." Initially, he claimed that they were "off in Africa or something," making wildlife documentaries, and this was why the protagonist had grandparents for roomies. But when kids started writing in asking what had happened to Arnold's parents, Bartlett wrote the Season 5 episode "Parent's Day." In it, we learned that his parents go on a humanitarian mission to the jungle and are never heard of again. (It's presumed that their plane crashed, though Bartlett leaves it deliberately open, since their plane was never actually found.)
He had further plans for the storyline and even hatched a movie plot where Arnold would attempt to go to Central America and find his parents. Sadly, this film never got made at the time (at the time being the key word here). However, one Hey Arnold! fan theory maintains that we've been being hoodwinked this whole time. Phil and Gertie aren't grandparents, but parents who had their offspring at an advanced age. And that offspring is Arnold.
According to Creepy Pasta (quoted via teen.com, since the original Creepy Pasta page on the theory has since been deleted):
If you've watched "Parents Day," then you'll know this actually does partially work as a theory. The episode opens on Arnold being regaled by elaborate bedtime stories about his parents — about their adventures in the jungle and time as acrobats in a circus — which makes the supposed "true" story all the more suspicious. Like the other anecdotes that Phil tells, it's pretty out there and doesn't entirely add up.
According to the "grandpa," Arnold's dad really was a scientist and he really did meet Arnold's mother in the jungle. But when his dad's friend Eduardo came to visit when the couple were back in America, he said he needed their help since his tribe was dying out of a mysterious illness. They then take a plane into the jungle in the rain and are never heard of again. But at the beginning of the "true" story when we get what looks like a flashback, it's implied that Eduardo thought that Arnold's father always has his head in the clouds and that he was romantic and impractical. This is reinforced by the supposed father constantly making comments about the sky and falling down rock faces, thus requiring medical attention. It's hard to imagine that Eduardo would voyage to an entirely different country to seek salvation at the hands of the man he seemed to view as pretty incompetent.
And then there's the strange glee that Arnold's grandparents have when, after a night of dreaming about all the ways they support him, Arnold suggests they go back to the Parents' Tournament being hosted by his school. "Really? But I thought you said that was for actual parents?" asks Phil. Arnold replies "For me, that means you and grandma." Gertie is so euphoric that she clambers onto the kitchen table.
There's also that hydrocephalus angle, which provides one explanation for why Arnold and Arnold alone (suggesting a characteristic specific to him, rather than a quirk of the cartoonist's style) has such a wide head. According to webmd.com, the main thing to look for with hydrocephalus is a "larger than normal" head.
Still, the medical aspect of the theory also undermines it. According to Australian charity The Brain Foundation if you don't treat hydrocephalus, then it can cause disability or death; health24.com supports the idea that hydrocephalus requires treatment, stressing the importance of medical support if you suffer from the issue. It seems highly unlikely that Arnold had the disease and wasn't being treated for it and this was no big deal. After all, he seemed perfectly healthy.
So, no, this isn't the world's most convincing explanation for where Arnold's parents were. But for an idea you might want to unpack over a couple of beers with your similarly Hey Arnold! obsessed best friend, if only to blow their mind? Unbeatable.