What is the role of pituitary in causing bitemporal hemianopia?

Proximity to optic: The pituitary gland sits immediately behind the optic chiasm which is where the optic nerves from each eye cross. A pituitary tumor or other swelling there of can press on the optic chiasm and cause one to lose vision to each side. Anuerysms of the anterior communicating artery can also do this. Very cool question. One would usually need an MRI of the brain to furhter investigate.

Optic chiasm : The pituitary sits near the optic chiasm. This area is where the optic nerve tracts cross. When there is enlargement of the pituitary or an adenoma, this can put pressure on the chiasm and cause bilateral hemianopsia.