It can definitely be the reason you are having abnormal periods. Some tumors increase the levels of prolactin, which can inhibit the ovulation and the whole period. Others can decrease FSH and LH and can affect ovulation as well.
I can review your case free of charge if you would like,
Pituitary tumors would be located on the pituitary, not the brain. They do impact hormones and which hormones are often determined by the type of tumor so you should find a competent endocrinologist - best a neuro-endocrinologist at a pituitary center (try a university or large hospital) and get the testing you need to determine the type of tumor so the course of treatment can be determined.
As a pituitary patient myself, I advocate getting copies of everything from MRIs to reports to labs etc as some doctors and labs are better than others. Lab error actually kept my diagnosis at bay for years and I did not know it until I did some research and met up with people smarter than me and finally realized I needed to learn. Time of testing is also critical. So you have to know things to know if your doctor is good and read the reports so you know if the doc is saying all is ok is really true (many of us found that getting the labs showed things were not always really ok).
Things can be resolved - and many lesions can be helped with medications. It seems very scary in the beginning. Hopefully learning will help. There are links in the health pages in the patient forum that may help you as well.
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