Sounds like your iguana needs to see a herp vet right away. Bring a stool sample with you and they can check it for parasites and treat him if need be.
Weight loss is a symptom of something very wrong and it's best not to wait it out.
What are you feeding him?
What type of lighting/heating does he get?
Any calcium/vitamin supplements?
How is he cranky? Is he swatting at you with his tail? Biting?-This could be hormonal/territorial or he could just be stressed from not feeling well.
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Christina Borders

