Terminology
In diagnosis, primary polydipsia is usually categorised as:

Psychogenic (PPD) – caused by underlying psychiatric symptoms, including those caused by psychoses and rarely by affective disorders
Non-psychogenic – another non-psychological cause, including idiopathic (unknown cause)The terms primary polydipsia and psychogenic polydipsia are sometimes incorrectly used interchangeably – to be considered psychogenic, the patient needs to have some other psychiatric symptoms, such as delusions involving fluid intake or other unusual behaviours. Primary polydipsia may have physiological causes, such as autoimmune hepatitis. Since primary polydipsia is a diagnosis of exclusion, the diagnosis may be made for patients who have medically unexplained excessive thirst, and this is sometimes incorrectly referred to as psychogenic rather than primary polydipsia. Non-psychogenic
Although primary polydipsia is usually categorised as psychogenic, there are some rare non-psychogenic causes. An example is polydipsia found in patients with autoimmune chronic hepatitis with severely elevated globulin levels. Evidence for the thirst being non-psychogenic is gained from the fact that it disappears after treatment of the underlying disease. Non-human animals
Psychogenic polydipsia is also observed in some non-human patients, such as in rats and cats. See also
Water intoxication
Fluid deprivation test

References
Further reading
Siegler EL, Tamres D, Berlin JA, Allen-Taylor L, Strom BL (May 1995). "Risk factors for the development of hyponatremia in psychiatric inpatients". Archives of Internal Medicine. 155 (9): 953–957. doi:10.1001/archinte.1995.00430090099011. PMID 7726704. Mauri MC, Volonteri LS, Fiorentini A, Dieci M, Righini A, Vita A (July 2002). "Efficacy of clozapine in a non-schizophrenic patient with psychogenic polydipsia and central pontine myelinolysis". Human Psychopharmacology. 17 (5): 253–255. doi:10.1002/hup.407. PMID 12404683. S2CID 21589725.
However, these changes will likely only develop after chronic PPD associated with severe mental illness, as opposed to less severe forms of the disorder as seen in those with anxiety and affective disorders. PPD is also linked with significant reductions in insular cortex volume, although this may be caused by the secondary hyponatraemia. It has been suggested that these deficits lead to moderate to severe cognitive impairments, especially affecting working memory, verbal memory, executive function, attention and motor speed.Other areas with volume reductions (both white and grey matter) include:
Right posterior lobe
Right inferior temporal gyrus
Parahippocampal gyrus
Both left and right superior temporal gyri
Right cuneus
Left medial frontal gyryus and inferior frontal gyrus
Right lingual gyrus

Diagnosis
As a diagnosis of exclusion, a diagnosis of primary polydipsia may be the result of elimination of the possibility of diseases causing similar signs and symptoms, such as diabetes insipidus. Diagnosis may be complicated by the fact that chronic and extreme compulsive drinking may impair the response of the kidneys to vasopressin, thus reducing the kidneys ability to concentrate the urine. This means that psychogenic polydipsia may lead to test results (e.g. in a water restriction test) consistent with diabetes insipidus or SIADH, leading to misdiagnosis.Dry mouth is often a side effect of medications used in the treatment of some mental disorders, rather than being caused by the underlying condition. Such medications include antipsychotics, antidepressants, anticonvulsants, alpha agonists and anticholinergics. It should also be ensured that the thirst isnt caused by diuretic use (particularly thiazide diuretics), MDMA use, excessive solute intake or chronic alcoholism.