Why is early diagnosis of pituitary tumors such a problem?

Few symptoms. Pituitary tumors are slow-growing and tend not to cause any acute symptoms, allowing them to grow to a relatively large size prior to their discovery. Furthermore, most do not cause hormonal problems, and the usual symptoms they cause (vision change, sometimes headache) are nonspecific and may be caused by a number of other conditions.
Too small and deep. The pituitary is located in a boney saddle deep inside the skull. The MRI is best to visualize it, but it is still hard to evaluate. Hormone and endocrine studies may help, but can be very difficult to interpret. Small changes in the appearance can be normal or tumor and still look the same. Simple biopsy is not possible.