My first dog, Patches developed Hypothyroidism very early in life (we suspect) but we had an incompetent vet, who didn't believe us that Patches had been on a diet since before she was 2! He thought we fed her too much/too many treats etc... It took 'till her eye bulged out for him to believe there was anything wrong (she was 5 years old at the time)!!! He even miss diagnosed a goiter as a pulled muscle in her neck when she was only a little over a year! (well, I assumed he did, as she had all the symptoms and developed Hypothyroidism after...)
When she was finally diagnosed, she was in the vets for more than a week, and very nearly died... all because our vet did not believe us, and obviously did not know the signs of HT. UGGH! Patches never totally lost the nearly 20 extra pounds she was carrying, though she did lose a good portion of it. And had she been diagnosed earlier, she probably would not have had the Arthritis issues she started developing quite young.
BUT Patches lived to be 14.5 years, and we were amazed she lasted as long as she did. We never expected her to live past 10 or 12, as unhealthy as she was.
Here's my baby (the only pic I have on my computer!) a few weeks before I had her PTS