A 28-year-old man presented to his optician with a 6-month history of gradually deteriorating vision. He was found to have a visual field defect and was referred urgently to an ophthalmologist who confirmed the finding, and arranged the following investigations.
Investigations (at 09.00 h):
serum cortisol300 nmol/L (200C700)
serum testosterone6.5 nmol/L (9.0C35.0)
plasma follicle-stimulating hormone1.2 U/L (1.0C7.0)
plasma luteinising hormone1.3 U/L (1.0C10.0)
serum thyroid-stimulating hormone2.4 mU/L (0.4C5.0)
serum free T411.1 pmol/L (10.0C22.0)
What is the most important next investigation?
A . adrenocorticotropic hormone
B . insulin stress test
C . prolactin
D . random growth hormone
E . short tetracosactide (Synacthen®) test
Answer: C