HYPOTHYROIDISM/THYROID DISEASE IN PETS

As with humans, it's always about cortisol and T3, (triiodothyronine). The reason animals become ill is because these two are not working properly in the system or out of balance and causing sickness. It's not as difficult as you'd think to diagnose correctly, but you've got to have the right blood labs done and you need someone intelligent and competent enough to read them.
(I highly recommend you buy a book for your Vet as well.)
Jean Dodds, DVM has just put out a book which is particularly good for beginners to study. I highly recommend this as well. We have a video of her discussing thyroid-ology in pets, take a listen to this:
Video:

Book:
Age has nothing to do with hypothyroidism, not in people, not in pets.
If you'd like to give your pet the best care possible, do the research and vow to understand it inside out.
They'll love you for it and they'll be around a lot longer to tell you so !