Endocrinology

Question 1

Primary polydipsia most likely to occur in what population

Answer 1

middle aged females with psychiatric illness

Question 2

Explain plasma osmolality, plasma ADH and urine osmolality in primary polydipsia (psychogenic polydipsia)

Answer 2

As the person drinks more and more water the plasma osmolality will go down (less solutes in plasma due to increased water); With a decrease in plasma osmolality there is no stimulation on the posterior pituitary to release ADH therefore serum ADH is decreased. Given large amounts of water this person is drinking the urine osmolality would also expected to be diluted (low)

Question 3

What causes ADH to be secreted from the posterior pituitary

Answer 3

An increase in plasma osmolality > 295 mOsm/L

Question 4

If a person with primary polydipsia received a vasopressin water deprivation test what would you expect to happen?

Answer 4

The pt has low plasma osmolality because of increased water intake. If the patient were water restricted and given vasopression (ADH) then plasma ADH would increase causing the pt to retain more water and they would concentrate their urine more appropriately

Question 5

If a person with diabetes insipidus received a vasopressin water deprivation test what would you expect to happen?

Answer 5

If the patient were water restricted, and then given exogenous ADH if they have neurogenic DI then you would anticipate urine, in a vasopressin water deprivation test, to be somewhat concentrated still a decreased response (partially responds to vasopression ~ 50%). Nephrogenic DI is caused by a deficiency or total resistance to vasopressin. So in the water challenge the pt would not be responsive to the vasopressin. While they were water restricted giving additional vasopression plasma and urine osmolality would still remain low

Question 6

What are the differences you would expect to see in a pt with SIADH versus primary polydipsia?

Answer 6