"a tumour on your pituitary gland"

animation by Anatomography

The pituitary gland secretes hormones that regulate homeostasis (maintaining the body in a state of equilibrium).  Pituitary tumours commonly secrete excessive growth hormone or prolactin, which stimulates lactation.  They are mostly benign, but their position close to the centre of the brain can make them dangerous.

Brain Surgery.com: Pituitary Tumours

National Cancer Institute: Pituitary Tumours