if you can't think straight, you are not thinking calmly and clearly about something.

I was so tired I wasn't thinking straight any more.

There are so many people talking, I just can't think straight.

• run your eye over something
• run your mouth
• run yourself into the ground
• rush of blood
• rush of blood to the head
• rustle up
• at first hand
• at first sight
• at full pelt
• at full steam
• at full stretch
• at full throttle
• at full tilt
• at hand
• at heart
• at heels
• a thick skin
• a thing of the past
• a thing or two
• a think-tank
• a thin line
• a thin skin
• a third wheel
• at home
• a thorn in someone's side