what does Dr. Jekyll's statement "It will not last long, thank God" foreshadow?

This statement foreshadows Jekyll's complete disappearance and freedom from the torture of Hyde. He knows that he won't be able to fight Hyde's complete control of him for much longer, and in a huge sense he's thankful to give up.

"I am very low, Utterson," replied the doctor drearily, "very low. It will not last long, thank God."