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Disease: Syndrome of Inappropriate Antidiuretic Hormone Hypersecretion (SIADH)
Rare form of hyperfunction of the PP, producing excessive level of vasopressin (VP) that is inappropriate for plasma osmolarity in individual
What causes an individual to develop Syndrome of Inappropriate Antidiuretic Hormone Hypersecretion (SIADH)?
most likely not actually a pituitary problem - often caused by ectopic (in an abnormal place or position) source of VP (e.g. lung cancer)
can occur after AP pituitary surgery (Initial diabetes insipidus (low VP) is followed by SIADH (high VP) which shouldbe managed with reduced fluid intake to prevent hyponatremia
Syndrome of Inappropriate Antidiuretic Hormone Hypersecretion (SIADH)
Effects on kidney, urine, and plasma?
Water reabsorption by kidney is increased and leads to a reduced renal volume and more concentrated urine
consequence is hyponatremia (decrease in plasma sodium concentration)
Diabetes Insipidus (DI)
failure of the posterior pituitary to produce vasopressin (VP, ADH)
presents as a polyuria (excess urine) and polydipsia (consequential excess drinking), so it's important to rule out diabetes mellitus
What can cause diabetes insipidus?
- Post-pituitary surgery that has caused some form of damage to the posteriorlobe
-Metastases and posterior lobe failure
-Fractures of the skull
-End-organ kidney failure
-Transient after transphenoidal pituitary surgery
How is diabetes insipidus diagnosed?
fluid/water deprivation test with hourly measurements of urine volume and osmolarity
(Monitor for dehydration due to large water volume loss)
What is the treatment for DI?
Use of a VP analogue (that contains a D- rather than an L- form of arginine in themolecule) to provide a longer half life than VP
Oxytocin deficiency
only a problem in breast feeding patients and these individuals can be given oxytocin analogues
Oxytocin excess
Excess production of oxytocin (e.g. ectopic tumor) is rarely seen as it won’t produce symptoms
Nonapeptides are transported to their target tissues by which 3 mechanisms?
1. Release of hormones into the blood occurs either via the general circulation fromthe PP or into the hypothalamic-hypophysial portal system (and AP)
2. Release at nerve endings across synapses within the CNS where the nonapeptidescan act as neurotransmitters
3. Release from dendrites terminating at the 3rd ventricle which can carry the hormones into the CSF for transport to central receptors
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