Look at the report or the mri yourself and see what the slice thickness was as it is usually 3-5mm and a 4mm tumor can fall between the slices.
Prolactin is not an easy hormone. It can be raised by thyroid, stress and so many things that it must be evaluated carefully.
The doctor saying that it is non-secreting like that is nothing is actually quite the opposite as those tumors grow quickly and require removal. So it is quite obvious that this doctor is not skilled in pituitary and you must find another.
Pituitary mri is a dynamic method and requires contrast and that the contrast uptake be recorded.
Find a pituitary center.