I had a customer order a radiator from me a while ago (to suit a 3S-GE).
Exactly the same thing happened to him as to you. When I got over to his place I removed to radiator cap and replaced it again, refilled the over flow bottle and then went for a drive.
From what I can see these engines develop air pockets and this appears around the temps sender for whatever reason.
Once we took it for a drive everything was back to normal. Just make sure you have connected all the appropriate sensors back up correctly and in the correct places.
It may sound like crap but it worked in this case with the same car, engine and customer twice.