> I think you and I would agree that this is a circular argument and simply shows that many of the tests, even now, are not sensitive or detailed enough?

I think not detailed enough, definitely.  I would argue that it's actually underdiagnosed because of that - far too many vets (the vast majority from my experience) still believe that running just T4 or TSH is fine for a definitive diagnosis and to be blunt, it's complete cr@p.  I must say I'm glad to see that the Merck does say that TSH in 20-40% of dogs remains within range even when hypothyroid - I may just print that bit out for my vets!  Totally unreliable test (in people, too).

The other part of the problem is that vets - like most human doctors - don't understand the significance of a RANGE for a particular factor, and assume that as long as the patient is somewhere inside that range that they are healthy, when that's simply not the case.  It's certainly not for me, and it isn't for Paige, and it wasn't the case at all - totally, utterly the opposite - for Remy for a full 3 years of very clear symptoms before he finally dropped off the bottom end and got diagnosed.   In that time he had numerous T4 and free T4 tests and at least 2 TSH tests (including one full stimulation test) and all were within range - but he was very clearly, obviously hypothyroid.  They just don't know how to read/interpret the results properly.

> Also thyroid hormones are different to sexual hormones so no, it's not a natural progression that a neutered animal will get hypothyroidism.

As estrogen enhances the ability of thyroxine to affect our bodies, in humans anyway, this seems entirely possible to me.

> I've had 12 dogs now, all of them neutered at various ages, and only 4 of them have gone on to be hypothyroid

That looks like a very high percentage to me, are we talking males though?

I am interested to know whether your Dobermans with thyroid problems suddnely became aggressive, as did my spaniel. 
BTW the spaniel is taking 2 and a half of these pills = 375 mcg daily.

Aggression and or anxiety are just two known behavioural changes that can occur with thyroid problems.

To the OP how are things now?

Can it be argued then that cocker spaniel "rage" is essentially a thyroid problem that could be inherent in many cocker spaniels?

If it was purely a thyroid problem then it's unlikely to be a breed-specific problem (that is much, much rarer than it was) because any breed can have thyroid problems and don't suffer from 'rage' syndrome.

> I am interested to know whether your Dobermans with thyroid problems suddnely became aggressive, as did my spaniel.

My dobes didn't show aggression but my rott X, Opi did - the first sign I had that anything was wrong was her becoming aggressive with other dogs where she has always been exemplory with them before, to the point of being my best stooge dog.

375mcg still doesn't sound like a massive dose to me - but response/dose does vary a bit with the product.

> That looks like a very high percentage to me, are we talking males though?

But as I said, 3 of the 4 dogs (one male) are dobermanns, which are naturally very prone to the disease.  1 out of 8 remaining dogs is not a high percentage IMO (and of the 8, only Linc who is half dobe has any hypo-prone breed in them).

Of course how you define hypothyroidism does make a difference - Remy was not considered technically hypothyroid by any vet for 3 years and Paige still isn't, because they were/are both within the lab ranges.  But symptomatically, both are hypo.

I'd be asking to see an eye specialist - they can see a whole lot more than a bog standard vet, I would also be doing blood tests. I don't know much about Cockers, but I am aware of Rage Syndrome. I'd want that ruling out. Are you insured? I guess your answer may well have an implication on how far you're willing to go with testing.

Hopefully your boy has turned into a Grumpy Old Man, but any sudden character change would be ringing alarm bells for me, and I'd be wanting to rule out all health causes.