The most common causes are pituitary tumor, thyroid imbalance or medication side effect (if you are not pregnant).
Do know that up to 40% of pituitary tumors do not show up on MRIs so having a normal one may not put you in the clear. My teeny little one hid for 12 years.
If it is a tumor, it is most commonly treated with medication, not surgery. Make sure your doctor tests all your hormones, not just a couple. A neuro-endocrinologist is best.