Jeez Ed. I feel for you. I just went through a nightmare with my 15 year old boy on tapazole as well. He just fell apart and it took 7 days to 'get him back'. I thought he was done. He's great now. He also started with not eating or drinking.
In my case we (me and my wonderful vet) could only conclude, after all that dust had settled,  that he wasn't getting his medicine everyday.

I had always put it in his food. Crushed to powder and mixed in well so his first bite is not Tapazole (he hates the taste and will walk away if he tastes it). But I wasn't paying enough attention to wether or not he was eating it all. So I think he was caught in a cycle of less and less food = less and less medicine. And when the thyroid is ailing, it affects every bodily function.

Anyway, (I'm not suggesting this...this is just my story) while he was dieing for a week in my arms, he seemed to like and be able to drink Hood 2% milk. My vet said that's ok seeing how it was the only 'food' he could deal with. She also said that milk is a laxative for them and it would be ok under the circumstances. No food other than that was he able to eat for 6 days! The milk kept my little sweetheart alive. And I had to administer 7.5mg  a day of tapazole directly down his throat.
He was not happy about that but that's what saved his life. On the 7th morning he ate some fresh turkey breast and then 15 minutes later I set out the food for my two boys (him being one of them) and he ate every bite since then. Whew!  He was a goner. He decided to stay. On my birthday no less...best present I ever got!  

I've gone back to 2.5mg of Tapazole (vet ordered) powdered in his food again. At times (not many anymore thankfully) when he either is just not hungry enough or for whatever reason, he doesn't eat much at medicine time, what I do is take that fresh turkey breast (I always have some for kitty snacks), put some Tapazole powder in a small plate, and 'dip' the pieces in very small amounts of the powder one after another until he gets it all.  He tastes the Tapazole and gags sometimes (he really hates it) but he can't resist the turkey. Bottom line is I have to be vigilant and make sure he gets his tapazole everyday.  By hook or by crook. He's beautiful inside and out again. And so much closer to me now for saving him. He was my little sweetheart before this happened. He can't be with me enough now. A blessing to me in every way.

Now tell me,  Iko's T4 count (thyroid unit) is ok? Do you know what the number is (T4)?
What food are you feeding him and how are you administering the Tapazole?
I'm certainly no expert but I can share my experience with you if that will help.
-R