ADH

Question 1

Where is ADH secreted?

Answer 1

Posterior pituitary gland

Question 2

What is the function of ADH?

Answer 2

Vasopressin

Stimulates reabsorption of water in the collecting ducts

Question 3

Causes of SIADH

Answer 3

Posterior pituitary secreting too much ADH
ADH may be coming from somewhere else - SCC

Post-operative from major surgery
Infection, particularly atypical pneumonia and lung abscesses
Head injury
Medications (thiazide diuretics, carbamazepine, vincristine, cyclophosphamide, antipsychotics, SSRIs, NSAIDSs,)
Malignancy, particularly small cell lung cancer
Meningitis

Question 4

What happens in SIADH?

Answer 4

Excessive ADH = excessive water reabsortion
Water dilution = euvolaemic hypoantremia
High urine osmolality
High urine sodium

Question 5

Symptoms of SIADH

Answer 5

Non specific

Headache
Fatigue
Muscle aches and cramps
Confusion
Severe hyponatraemia can cause seizures and reduced consciousness

Question 6

Diagnosis of SIADH

Answer 6

Other causes of hyponatremia need to be excluded

Negative short synacthen test to exclude adrenal insufficiency
No history of diuretic use
No diarrhoea, vomiting, burns, fistula or excessive sweating
No excessive water intake
No chronic kidney disease or acute kidney injury

Question 7

What is Central pontine myelinolysis (CPM)?

Answer 7

complication of long term severe hyponatraemia (< 120 mmols/l) being treated too quickly (> 10 mmol/l increase over 24 hours)

Question 8

What is diabetes insipidus?

Answer 8

Lack of antidiuretic hormone (ADH) or a lack of response to ADH

Prevents the kidneys from being able to concentrate the urine leading to polyuria and polydypsia

classified as nephrogenic or cranial

Question 9

What is nephrogenic diabetes insipidus?

Answer 9

Collecting ducts of the kidneys do not respond to ADH

Also caused by:
Drugs, particularly lithium used in bipolar affective disorder
Mutations in the AVPR2 gene on the X chromosome that codes for the ADH receptor
Intrinsic kidney disease
Electrolyte disturbance (hypokalaemia and hypercalcaemia)

Question 10

What is cranial diabetes insipidus?

Answer 10

Hypothalamus does not produce ADH

Brain tumours
Head injury
Brain malformations
Brain infections (meningitis, encephalitis and tuberculosis)
Brain surgery or radiotherapy

Question 11

Presentation of diabetes insipidus

Answer 11

Polyuria (excessive urine production)
Polydipsia (excessive thirst)
Dehydration
Postural hypotension
Hypernatraemia

Question 12

How is diabetes insipidus diagnosed?

Answer 12

Water Deprivation Test
Also known as the desmopressin stimulation test

8 hours fluid deprivation, urine osmolality is measured and synthetic ADH (desmopressin) is administered. 8 hours later urine osmolality is measured again

Question 13

Water Deprivation Test Findings

Answer 13

After Deprivation
After ADH

Cranial Diabetes Insipidus
Low
High

Nephrogenic Diabetes Insipidus
Low
Low

Primary Polydipsia
High
High