Were there any clues that showed that the Shredder wasn't really human

[PApagreg]

I was just wondering if they had any clues

[CyberCubed]

At first no. He always did have his stomach bandaged up when he had his clothes off though.

[ToTheNines]

His Stomach was never shown. He could hold the Sword of Tengu with no consequence. And he got his head chopped off :)

[Technogeek29]

That's actually something the OT & 4kids share. Shredder is responsible in so form for their mutation. Less so in the Nick verse however.