One Droopy Eye, Pituitary Gland? (photo)

Though I agree in theory with Dr Steinsapir, but in medicine never say never. Your history can lead one to think the tumor "might" be the issue. But unless there has been an increase in its size, than hard to point a finger at the tumor. Unless it is NOT an adenoma but a different type of tumor, one that causes secretions of hormones or precursors. Also the pressure of this tumor in the area of the pituitary gland could cause the optic nerve and occulomotor nerve issues, as a slim possibility. Best to obtain results from MRI and see neurosurgeon, neurologist, oculoplastic surgeon. Please type us follow up. 

Microadenomas are pituitary adenomas are less than 10 mm in size.  At 2.5 mm yours is small.  They are typically diagnosed due to an incidental imaging study of the brain, or it they secrete hormones that affect the body in some way.  These smaller lesions are not typically associated with any visual symptoms.  The macroadenomas on the other hand can impair vision because they grow to a size where they impinge on the optic nerves.  It sounds like you are being followed very closely.  In your photo, you do have mild left upper eyelid ptosis.  It is unlikely that this is related to the pituitary tumor.  However, these is a reasonable questions.  The best specialist to see for this will be a neuro-ophthalmologist.  Your endocrinologist can likely make this referral for you.

These can be entirely unrelated issues or they can be related to eachother. I'd advise that you see an ophthalmologist who specializes in eyelid surgery (oculoplastic surgeon) to sort this out.