I had a similar circumstance. The first, simple test done showed that I had the kidneys of an 80 year old woman! So they did the glofil test. Not a problem - it just takes a long time as there are a number of blood draws involved to monitor the progress of the test. They will insert an IV, then put in the radioactive tracer, and you will drink a small cup of liquid that blocks your thyroid from absorbing the substance. The regular blood draws are not a problem, as they draw from the IV, but you just are there for a period of time - bring a book or something! The results of that test showed my kidney function was fine.
The center indicated that the glofil is a much more accurate test of kidney function that some of the other tests. I would not worry about it at this time.