Indoors or out?
Yellow leaves, with a Meyer, almost always mean that it needs Nitrogen....lots of Nitrogen. Could also be overwatering, if you're sure that you're feeding it regularly and sufficiently.

Josh

OK, thank you. I'll feed more. Will stick my hand way down the into pot and see if it's too wet. I have never had a lemon do this before. I thought it was the iron because my healthy gardenia is losing leaves too.

Yellowing leaves occur for *many* reasons so it's iffy to accurately determine the correct diagnosis without an image.

Please post a picture of the entire shrub as well as a mid-range image of an affected area.