Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Call emergency services for a sudden severe headache accompanied by any of the following:
These symptoms can have several causes, including pituitary apoplexy, stroke, meningitis or bleeding around the brain.
They need immediate assessment rather than a routine appointment.
Do not drive yourself to hospital.
If you have already been diagnosed with pituitary apoplexy, seek urgent help for worsening vision, increasing sleepiness, persistent vomiting or symptoms from your adrenal emergency plan.
These can be reviewed through routine pituitary follow-up.
The article also says
Contact your usual healthcare team for non-emergency concerns such as ongoing fatigue, thirst, frequent urination, menstrual changes, sexual difficulties, medication questions or gradual changes in headache.
Every sign above is quoted from the sofidex article on Pituitary apoplexy. If you are not sure, err upward — it is always reasonable to be checked.