An even more alarming thought followed: What if my entire intellect is disintegrating?

There was an alarming thought, and now that she came to think of it, that made perfect sense.

By now, Gel had gotten his second wind, and such alarming thoughts only increased his energy.

This is an alarming thought because it may be true.

Then he had an alarming thought: What had he been like back in college?

The alarming thought is that a "species barrier" has been crossed and may be no longer protecting humans.

To Data, that was a most alarming thought.

It was an alarming thought, especially considering what she knew of recent imperial ship movements.

But as he considered this, an alarming thought came to mind.

An alarming thought had come into his head.