A healthy tegu will drink plenty of water daily, with plenty of urine as a result. You can check your tegu’s hydration by pinching skin lightly: If it is healthy, the skin should fold back into place. If it stays pinched, your tegu is dehydrated.
For moderate dehydration, soak the tegu in a 50/50 solution with Pedialyte. Unflavored Pedialyte is best, but flavored may encourage a reluctant tegu to drink. Also make sure that the tegu has access to clean, fresh water every day.
If your tegu does not improve, it is likely severely dehydrated and requires immediate veterinary intervention. Do what they say, regardless of cost. Dehydration is a slow, painful killer when left untreated.