I knew this post would get interesting. Thank you Amy for finding that information!

A picture really is worth a thousand words. Seeing this beautiful coloring is one thing but seeing in the last pics, the close up of the guard hairs, that's just wow. You really can see something isn't quite right in that pic.

@aljones- yes I'm basing health of the overall coat on the complete color change. I know it's not uncommon for Siberians to change as they grow, we've all seen that to one degree or another. What we do not see is an otherwise healthy animal decide it's going to become a completely different color (this is not the same as a dark red turning light red or a buffy puppy resulting in a grey adult). It would be impossible for a dog to re-write it's genetics to become a completely new color. It is not remotely natural for a black coat to change color to this degree- this indicates there is either something very deficient in diet (that would be extreme and you'd expect other side effects), damage, or something impacting over all health of the animal. Black hair turns red when it dies. Sun damage, dietary deficiencies, and overall health of the animal (remember that healthy coat starts from within) speeds the process. Ultimately though what is being seen is a broad spectrum coat death. That shouldn't occur in an otherwise healthy coat. Reading the posts FD should also be looked into, but I wouldn't completely rule out thyroid. Siberians thyroid levels are already one of the lower levels of dog breeds, running a breed specific thyroid panel couldn't hurt, you may not see other symptoms but you are seeing something rather concerning.

HuskyMom09 wrote:

To further illustrate-  this would be a dark sable coat



The above dog as a puppy, you can see from birth this would not have been confused as a b/w-




A light sable coat


Close view you can see how the sable coat is a red/tan base to a black tip (vs a true 'red')-




This is in effort to illustrate Sable is not a 'red coat with black pigment'!!

Are these yours, Lani? They are beautiful.

Thanks everyone for the posts and comments, they have all been a great help. I actually had already reach out to Kato's Vet for some answers since I assumed maybe it was something I did or didnt do that cause his coat changed. He already scheduled for testing for the three things mentioned in the op (cushings, thypothyroidism, and alopecia). I figured I would come here because someone would have either seen something like this before or know what is happening and I think we def have a lock on the FD diagnosis.

I just hope my poor boy doesn't lose his fur as a result of whats causing this. I will continue to keep everyone updated and have a phone call scheduled in the morning with the vet to discuss the follicle issue

I think this thread has been interesting for several reasons:
It's common knowledge that none of knows everything. We all know some things and some of us don't know as much as we'd like to think (that, BTW, is NOT directed at anyone in particular). My lack of understanding about an agouti led me down a path that also educated me - thanks @HuskyMom09 for all you did to teach me. Your experience is invaluable!
@amymeme did something that irritates me with "Check out follicular dysplasia", I didn't know enough about the subject for google to be any help, I didn't know what I was looking at / for. She saved herself, for me, when she provided a link later that put the spotlight on the issue. I very much appreciate that reference - even if I did feel like "Duh, I'm dumb!" after reading it.

I really hope @Arkentect follows through with diagnosis / prognosis and current state as time passes, I'm intrigued by this whole situation as she presented it.

Al... When I'm on my phone, keyboarding, copy paste etc exceedingly burdensome. And Chuck full NB of steps. See. Should say chick full of errors :p

Yeh, tell me. I have a "keyboard" on my tablet and I still won't respond to anything on it 'cause my fingers are too fat for the little keys there, I can imagine what it's like to try to type on a phone - NOT!!

Woah! That is seriously crazy. His puppy pictures remind me of my pup because she has reddish/brown spots in some areas of her coat but dang!! I've never seen anything like that, this thread was really interesting to read. Hoping you get a clear diagnosis from the vet and that everything will be ok!