Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
1
Is any of this happening now?
Call 911
Seek emergency help if absent thirst or poor fluid intake occurs with:
New confusion, extreme drowsiness, fainting, or a seizure
Inability to wake or respond normally
Severe weakness with little or no urine
Rapid worsening after a head injury
These symptoms may indicate severe dehydration, a major salt imbalance, or another serious problem.
Other emergency signs, for anyone
Someone who is struggling to breathe, gasping, choking, or cannot get words out
Someone who has chest pain, tightness or pressure lasting more than a few minutes
Someone who has sudden weakness or drooping on one side of the face or body, slurred speech, or sudden confusion
Someone who is unconscious, or cannot be woken
Someone who is bleeding heavily and it will not stop with firm pressure
Someone who has had a seizure that will not stop, or does not wake up afterwards
Someone who has lips or skin that look pale, blue, grey or blotchy
Someone who you believe has taken an overdose or swallowed something poisonous
2
Is any of this happening?
Be seen today
Arrange same-day medical advice if you are passing very little urine, becoming dizzy when standing, unable to keep fluids down, or getting steadily weaker.
Repeated vomiting, diarrhoea, or fever can increase fluid loss even when you do not feel thirsty.
3
Does any of this apply?
Book an appointment
Book a routine appointment if the change persists, repeatedly leads to low fluid intake, or began after brain or pituitary surgery.
You should also be assessed if absent thirst occurs with unusually frequent urination, large urine volumes, persistent headaches, changes in vision, or unexplained changes in alertness.
Every sign above is quoted from the sofidex article on Thirst - absent. If you are not sure, err upward — it is always reasonable to be checked.