Bicycle Mechanics - Left sided shifter for Tiagra double and triple the same or different?

I get different answers at the above questions. Are there different double and triple left sided shifter (for Tiagra), or is the same shifter compatible for either or ( I know higher end Shimano shifters have differnt types - this question is about Tiagra specific)
Thanks

Not sure if they are the same, but they are almost 99.9% double compatible if they're triple.

I had a Tiagra left shifter replaced on a double set-up. I believe the first shifter was triple-compatible. The one they replaced it with clearly says 'Double' on the gear pod indicator; the first one did not. So: I still can't answer your question, since I don't know if the internals are any different, but would also like to know the answer.

The Shimano site shows only the 4500 which is double/triple compatible: http://bike.shimano.com/publish/content/global_cycle/en/us/index/products/road/tiagra/product.-code-ST-4500.-type-st_road.html

Shimano has recently added a double specific shifter to the 105 Group. I suppose the same could be true for the Tiagra group but it's not shown at their site. Your LBS should know.

Origionally the 4500 was sold for both double and triples. Then Shimano introduced the 4501 for doubles and made a 4503 (which is basically the same as the 4500) marketed to triples.

This is my understanding.

Originally there was just 4500, which was marketed as double and triple compatible.

Then, 4501 (double only) and 4503 (same as 4500, but marketed as triple) replaced 4500. I believe this is because some people were breaking their shifter when they used the 4500 on a double setup when using the first two shift positions (inner and middle). Attempting to push the shifter into the outer position, which is locked out by the high limit screw, has the potential to stress and break the internal mechanism. Some people will set up the shifter so that the middle and outer positions are used, but this might mess up trimming.

In short, the 4501 can only be used for double. 4500 and 4503 are effectively the same and can be used for double or triple, but be careful if using a double.

`my bike is rigged with tiagra. Although is a double, my shifter is a triple one, so I guess they are compatible. Im not sure about what model is, but both of my shifters are indexed