Shock was the first thing that set in at entertainment towers this morning when we woke up to the sad news of Glee star Cory Monteith's death at the age of 31. It seems to have been the running theme in Hollywood too though: You need only look to Twitter for evidence.

From the moment the sad news broke, Monteith's co-stars, peers, and even those who had never known the young actor, took to all forms of social media to express their sadness. The loss of anyone in the business usually sparks a reaction, but Cory's premature passing has prompted quite the outpouring online.

Here's what the stars have been saying on Twitter:

It's important to stress that no cause of death has been determined, despite rampant speculation that the star may have died from an overdose.

One thing we can be sure of is that Monteith's passing has left the industry in a state of shock: Nobody expected the boy from Glee to end up in rehab, let alone dead at such a young age.