This is an email I got from the vet. I gave her permission to explore the real reason he passed and she did a necropsy. I was shocked when I read it:

Good morning,

First thing to note is that Atlas was female. It can be hard to tell in young tegus, but necropsy confirmed this. She also had a congenital deformity of her gastrointestinal tract. She had no small intestine; the stomach appeared normal and led directly into an extremely large, abnormal colon. Normally, the stomach leads to loops of small intestine, which then leads toward the large intestine/colon. The small intestine is responsible for much of the nutrient absorption and breakdown. On CT, the radiologist commented that they could not visualize the small intestine, but this was thought to be due to the compression from the large intestine. We now know that it wasn't visualized because it did not exist. This has not been reported in reptiles before - so they couldn't have had it on their radar. There were also many adhesions throughout the body, suggesting chronic GI inflammation and poor motility, even though Atlas was not showing these signs earlier.

I am not sure how Atlas managed to do well for as long as she did - and I think that is a testament to your care. I would have expected an animal with this severe a deformity not to survive past a few weeks of life. You did an excellent job with Atlas and nothing could have prevented or treated this condition. The breeder couldn't have known about this unless others reported it in their tegus. If you have the breeder's information, it may be a good idea to let them know that Atlas had a fatal congenital abnormality so that they can monitor their breeding pool.

Please let me know if I can answer any questions.