What is the most likely diagnosis?

A 56-year-old man was referred urgently by an ophthalmologist after presenting with a 6-month history of deteriorating vision. The patient had a 40 pack-year smoking history. Before his vision problem, he had never visited his general practitioner.

Investigations:

serum cortisol (09.00 h)389 nmol/L (200C700)

serum testosterone8.6 nmol/L (9.0C35.0)

plasma follicle-stimulating hormone2.1 U/L (1.0C7.0)

plasma luteinising hormone2.4 U/L (1.0C10.0)

serum prolactin896 mU/L (<360)

serum thyroid-stimulating hormone1.4 mU/L (0.4C5.0)

MR scan of pituitarysee image