Drastic loss of facial pigmentation

an otherwise healthy himalayan male, normally with dark pigmentation on face, ears, tail and paws..
Within 6-8 weeks he's lost 50% of the dark color around his mouth, nose and ears...
this week he's also losing the color on the tuffs of hair on the bottom of his back paws.

I know this normally happens GRADUALLY with OLDER Himalayans as it did with an older kitty I had before.
But its not normal to happen with one whose not even 7 years old yet and it wasn't a gradual change like I said previously it happened rapidly...all within a few weeks.

What do you think would cause this?
Would it be nutrition related since all his tests came back normal?
My Vet is stumped.

He has EGC that likely began from a chicken allergy(we did many changes 2 years ago)
He had to be OFF anything with chicken, refused venison. His main diet for all of 2010 was canned FISH (fancy feast I'm ashamed to admit). By Oct. 2010 to present he has remained FREE of the facial scabs he had been experiencing.

In January of 2011 I was able to get him transitioned to a better food...EVO by Innova 95% beef with no grains.
the pigment change started about the same time....however I didn't at the time think it had anything to do with his diet.(?)

The past 10 days since his tests all were normal my Vet recommended taking him off the EVO beef.
So right now he is eating a homemade pork diet...a recipe with all the recommended supplements as required.
I sprinkle a small amount of tuna on top to get him to eat it.

All I've been able to find on research is possibly a tyrosine deficiency could cause this in these Himalayan/Siamese breeds... could the EVO diet he was on for a few months be low in tyrosine?
I'm just throwing all this out to get your thoughts on how to stop this pigment loss and perhaps even reverse it?
thanks for the help..

[Aleda M Cheng, D.V.M., C.V.A]

I think that you have done your homework.  Your cat may have an unusually high requirement for the amino acid tyrosine and although Evo/Innova may have adequate amounts of tyrosine for the average cat it may be inadequate for your cat.  

I don't know if other commercial foods might have the same effect, so it is probably a good idea to feed your cat home cooked in which you can add additional tyrosine, if necessary as a supplement.  Tyrosine is available in capsule or tablet form online and at health food stores.

The following is an excerpt from the JAVMA (Journal of the American Veterinary Medical Association) article, "Timely Topics in Nutrition
The carnivore connection to nutrition in cats":  

Tyrosine, an amino acid that is not essential for other species, is considered to be conditionally essential for cats. It has an important role in the synthesis and homeostasis of melanin, which is found in black hair and skin pigment. Tyrosine is synthesized from phenylalanine, an amino acid contained in many proteins, but diets of cats may not contain quantities sufficient to support tyrosine and, subsequently, melanin synthesis. As a result, tyrosine deficiency is most commonly observed in black cats whose hair becomes reddish-brown.19 This effect can be reversed in cats fed diets that contain increased concentrations of tyrosine, which include diets high in animal-source proteins.19 Tyrosine is an excellent example of a nonessential amino acid that may become deficient in cats because of an increase in the use of tyrosine for production of hair or an increase in the use of its precursor, phenylalanine, for nonessential (degradative) functions.

For the complete article:
http://www.avma.org/publications/default.asp

Lastly, since tuna is high in mercury please try to use only occasionally (once weekly, if possible).  Try clam juice, fish juices from other canned sources such as mackerel, salmon, kippers or herring, shrimp, etc. to doctor her food.

[opus88]

thank you so much for the reply Dr Cheng.
I've just found out the results of his fT3 tests that were sent out to an independent lab.
T4    7.4 range(10-60)
fT3    3.8 range(9-33)

so he is indeed in the LOW range. my Vet still says its very unlikely for him to have hypOthyroidism, b/c this is very rare and he isn't taking any medication that would cause this disease.

to top off his problems now he is refusing to eat almost ALL the canned cat foods I can buy also the homemade diet even when I add tuna or salmon as an enticer. the only one he WILL eat is a junk food fancy feast(same problem I had when trying to transition him 2 years ago....thats why I was stuck feeding this junk)...

I am pulling my hair out trying to find something suitable that he will accept.

In your opinion is it still safe to add tyrosine for him?

Also what are your thoughts on giving him a herbal product called THYRO-PET?

http://www.nativeremedies.com/petalive/thyropet-healthy-thyroid-levels-cats-dogs.html

thank you so much for your help.