Endocrinology: Thyroid dysfunction Flashcards
What are the 2 mechanisms behind diabetes insipidus?
1) A lack of ADH (cranial diabetes insipidus)
2) A lack of response to ADH (nephrogenic diabetes insipidus)
What is diabetes insipidus caused by a lack of ADH also known as?
cranial diabetes insipidus
What is diabetes insipidus caused by a lack of response to ADH also known as?
nephrogenic diabetes insipidus
Where is ADH produced?
Hypothalamus
Where is ADH stored?
Posterior pituitary gland
What is ADH also known as?
Vasopressin
Function of ADH?
ADH stimulates water reabsorption from the collecting ducts in the kidneys.
Pathophysiology in diabetes insipidus?
1) Lack of ADH/lack of response to ADH
2) Kidneys unable to reabsorb water and conentrate the urine
3) Leads to polyuria and polydipsia
What is primary polydipsia?
Is this the same as diabetes insipidus?
Primary polydipsia is when the patient has a normally functioning ADH system but drinks excessive amounts of water, leading to excessive urine production (polyuria).
This is not diabetes insipidus.
What happens in Nephrogenic diabetes insipidus?
when the collecting ducts of the kidneys do not respond to ADH
Causes of nephrogenic diabetes insipidus?
1) Idiopathic
2) Medications, particularly lithium
3) Genetic mutations in the ADH receptor gene (X-linked recessive inheritance)
4) Hypercalcaemia
5) Hypokalaemia
6) Kidney disease (e.g. polycystic kidney disease)
What medication is most likely to cause nephrogenic diabetes insipidus?
Lithium
What happens in cranial diabetes insipidus?
Cranial diabetes insipidus is when the hypothalamus does not produce ADH for the pituitary gland to secrete.
Causes of cranial diabetic insipidus?
1) Idiopathic
2) Brain tumours
3) Brain surgery
4) Brain infections e.g. meningitis, encephalitis
5) Genetic mutations in the ADH gene (autosomal dominant inheritance)
6) Wolfram syndrome
What is Wolfram syndrome?
A genetic condition causing cranial diabetes insipidus, optic atrophy, deafness and diabetes mellitus.
Describe the inheritance pattern for genetic mutations in the ADH receptor gene causing:
a) cranial diabetes insipidus
b) nephrogenic diabetes inspidus
a) autosomal dominant
b) x-linked recessive
What type of diabetes insipidus can hypercalcaemia cause?
Nephrogenic
What type of diabetes insipidus can hypokalaemia cause?
Nephrogenic
What are the presenting features of diabetes inspidus?
- Polyuria (>3 litres of urine per day)
- Polydipsia
- Dehydration
- Postural hypotension
What are the 2 lab findings in diabetes insipidus?
1) Low urine osmolality (lots of water diluting the urine)
2) High/normal serum osmolality (water loss may be balanced by increased intake)
Also more than 3 litres on a 24-hour urine collection
What is the test of choice for diagnosing diabetes insipidus?
Water deprivation test (also known as the desmopressin stimulation test)
Describe the water deprivation test/desmopressin stimulation test
1) The patient avoids all fluids for up to 8 hours before the test (water deprivation).
2) After water deprivation, urine osmolality is measured.
3) If the urine osmolality is low, synthetic ADH (desmopressin) is given.
4) Urine osmolality is measured over the 2-4 hours following desmopressin.
In primary polydipsia, how will water deprivation affect urine osmolality?
Water deprivation will cause urine osmolality to be high (urine more conc).
Therefore, desmopressin does not need to be given.
In cranial diabetes insipidus, how will water deprivation affect urine osmolality?
What happens after desmopressin is given?
In cranial diabetes insipidus, the patient lacks ADH. The kidneys are still capable of responding to ADH.
1) Initially, the urine osmolality remains low as it continues to be diluted by the excessive water lost in the urine.
2) After desmopressin is given, the kidneys respond by reabsorbing water and concentrating the urine.
3) The urine osmolality will be high.