Med-Surg Endocrine System, part 10: Diabetes Insipidus and SIADH
August 11, 2020 Updated: January 17, 2022 13 min read 3 Comments
In this article, we cover two disorders, diabetes insipidus and syndrome of inappropriate ADH (SIADH). These are disorders that result from improper amounts of ADH in the body, so we begin with a quick review of ADH. Knowing the pathophysiology, symptoms, diagnosis and treatment for these two disorders will be key in your Med-Surg exams as well as your nursing practice.
These disorders are covered in our Medical-Surgical Flashcards (Endocrine system).
What is the difference between SIADH and DI?
Both diabetes insipidus and SIADH have to do with antidiuretic hormone (ADH). With diabetes insipidus, the body has insufficient ADH, and with SIADH, the body has excess (or an inappropriate amount of) ADH.
If you would like more in-depth information about ADH, we gave an overview of ADH in the pathophysiology section of this playlist.
Antidiuretic hormone review
As a quick overview, ADH is released by the posterior pituitary gland in response to:
- Low blood volume
- Low blood pressure
- Hypernatremia (increased blood osmolarity)
If the body senses any of these three things, it will release ADH from the posterior pituitary gland, which will cause the kidneys to reabsorb more water, which helps to:
- Increase blood volume
- Increase blood pressure
- Dilute the blood so the blood osmolarity drops to a normal level
Diabetes insipidus is sometimes jokingly called “the other diabetes,” meaning it’s not related to the much more common diabetes mellitus. The word diabetes comes from Latin and Greek meaning “siphon” or “to pass through,” referring to excessive urination common with the disease. The word insipidus comes from Latin meaning “tasteless,” referring to the diluteness of the urine.
Neurogenic diabetes insipidus
Neurogenic diabetes insipidus happens when there is some kind of trauma or tumor in the hypothalamus or pituitary gland which is causing insufficient ADH to be released. Without enough ADH being released from the posterior pituitary gland, the kidneys are not getting the signal to reabsorb water.
The prefix neuro- in neurogenic means nervous system, or more specifically, brain. The hypothalamus and pituitary gland are in the brain, so that’s how you can remember neurogenic diabetes insipidus.
Nephrogenic diabetes insipidus
With nephrogenic diabetes insipidus, the posterior pituitary gland is actually releasing the correct amount of ADH. But there’s a problem with the kidneys. They are not responding appropriately to the ADH signal. They are getting the ADH, but they don’t respond and don’t reabsorb more water.
Why don’t the kidneys respond to the ADH? It may be due to some kind of kidney infection or damage. Sometimes kidney damage occurs from use of nephrotoxic medications.
The prefix nephro- in nephrogenic means kidneys, so that’s how you can remember nephrogenic diabetes insipidus is a problem with the kidneys. Pretty easy!
If you’d like to learn more about prefixes and suffixes that can help you easily decode disease names, check out our Medical Terminology Flashcards.
Signs and symptoms of diabetes insipidus
One of the telltale symptoms of diabetes insipidus is large amounts of dilute urine. Dilute urine is urine that has a higher concentration of water than is expected -- in this case, because the kidneys are not reabsorbing the water. Dilution or concentration of urine is measured by the urine specific gravity test, which we will cover shortly.
Another key symptom of diabetes insipidus is polydipsia, which is excessive thirst. If you think about it, these two symptoms will exacerbate one another. If you are very thirsty, you will increase your water intake, but drinking tons of water will cause you to excrete large quantities of diluted urine, which can cause even more thirst in response to fluid loss!
Easy way to remember polydipsia
Diabetes inSIPidus will make you want to SIP more water because you're super thirsty!
Other signs and symptoms of diabetes insipidus include dehydration, hypotension, and loss of appetite.
Lab values associated with diabetes insipidus
Urine specific gravity
A urine specific gravity test measures the relative densities of a patient’s urine to the density of water. More specifically, it’s a ratio of the mass of urine to the mass of an equal volume of water, so it measures how dense or heavy the urine is compared to water. The closer the urine specific gravity ratio is to 1, the closer the urine’s density is to water.
The expected range for urine specific gravity is 1.010 - 1.025. Lower than that is dilute urine, and higher than that is concentrated urine. Very low urine specific gravity, under 1.005, can indicate diabetes insipidus.
A urine osmolality test measures urine concentration, or the amount of dissolved substances in the urine.
The expected range for urine osmolality is 300 - 900 mOsm/kg. Lower than that is dilute urine, and higher than that is concentrated urine. Very low urine osmolality, under 200, can indicate diabetes insipidus.
Serum osmolality (blood osmolality)
Blood (serum) osmolality is the amount of dissolved substances in the liquid part (plasma) of the blood. A large portion of these substances is sodium. Remember that one of the tasks that ADH gives the kidneys is to dilute the blood so its osmolality drops to a normal level. Without the kidneys doing that job, the blood becomes more concentrated and its osmolality rises.
The expected range for serum osmolality is 275-295 mOsm/L. Lower than that is dilute blood, and higher than that is concentrated blood. Very high serum osmolality, over 300, can indicate diabetes insipidus. This is opposite of the urine osmolality, because the fluid shifting into the urine results in excessively dilute urine, but significant fluid loss from the blood.
Sodium is an electrolyte that’s important for nerve and muscle function and maintaining fluid balance. Remember that a large portion of the substances in the blood is sodium — so as blood fluid levels drop and serum osmolality increases, sodium (Na) levels will also be high in a patient with diabetes insipidus.
The expected range for sodium (na) is 136 - 145 mEq/L. Lower than that can indicate hyponatremia, and higher than that can indicate hypernatremia. In diabetes insipidus, the expected sodium level would be above 145 mEq/L.
Having trouble remembering all these lab values? Urine specific gravity, urine osmolality, serum osmolality, and sodium are all covered in our Lab Values Flashcards. You can use these to practice for your nursing exams and as a reference guide if you’re a practicing nurse.
Diagnosis of diabetes insipidus
Water deprivation test
Remember that one of the symptoms of diabetes insipidus is producing large volumes of dilute urine. Well, large volumes of dilute urine can also be a result of drinking large volumes of water.
A water deprivation test basically checks to see what happens when you take the water away. Does the body behave normally when the water is taken away, or abnormally? Is the dilute urine due to too much water, or is the body actually unable to concentrate urine?
Normally, water deprivation would cause increased production of ADH, which would trigger the kidneys to preserve fluid, resulting in smaller volumes of more concentrated urine. But if the patient is deprived of water in this test, and still produces dilute urine, this is abnormal and can indicate diabetes insipidus
The vasopressin test helps differentiate between neurogenic and nephrogenic diabetes insipidus. Remember that neuro means brain (pituitary gland) and nephro means kidneys.
Vasopressin is a drug used as a hormone replacement for ADH. We expect it to do the same thing as ADH: trigger the kidneys to reabsorb water. If we give a patient vasopressin and their kidneys do not reabsorb water, we know it’s a kidney problem and we have nephrogenic diabetes insipidus. If we give the patient vasopressin and their kidneys successfully reabsorb water, then we know it was a problem with the pituitary gland not producing enough ADH, and we have neurogenic diabetes insipidus.
Treatment (medications) for diabetes insipidus
In the case of neurogenic diabetes insipidus, we can provide the patient medications like vasopressin or desmopressin (DDAVP) as an ADH replacement. Check out Cathy’s easy way to remember the side effects of antidiuretic hormones.
Nursing care for diabetes insipidus
When a patient has diabetes insipidus, you will want to montior their intake and output (I&Os), urine specific gravity, and daily weight. Weight is important because weight loss can occur with excessive fluid loss.
Also, monitor for signs of fluid volume deficit: tachycardia, hypotension, poor skin turgor, dry/sticky mucus membranes.
Syndrome of inappropriate ADH (SIADH)
Pathophysiology of SIADH
You can think of SIADH as basically the opposite of diabetes insipidus. With SIADH, there is excess secretion of ADH from the posterior pituitary gland.
Why does excess ADH get released? It can be due to a brain tumor, head injury, meningitis, or because of a medication. This excess ADH will be released even when serum osmolality is low (when the blood is diluted). This results in the kidneys reabsorbing more water — meaning the body retains too much water.
Signs and symptoms of SIADH
The key symptom of SIADH is a very small amount of concentrated urine. The body is holding onto the water so it’s not being released in the urine.
There will also be signs and symptoms of fluid volume excess. This includes tachycardia (fast heart rate), hypertension (high blood pressure), crackles, jugular vein distention, and weight gain. Some other symptoms the patient may have are headache, weakness, and muscle cramping.
With the blood diluted, this can lead to hyponatremia (abnormally low sodium), and one symptom of hyponatremia is confusion, especially in elderly patients.
Check out Cathy’s nursing tip for the easy way to remember SIADH symptoms!
Labs values associated with SIADH
Remember that SIADH is the opposite of diabetes insipidus. With DI, the patient has dilute urine and concentrated blood — with SIADH, the patient has concentrated urine and dilute blood
Urine specific gravity
With SIADH a patient has concentrated urine, so that means a high urine specific gravity — the urine is a lot denser than water. Urine specific gravity will be over 1.03.
This concentrated urine will also result in a high urine osmolality, over 900 mOsm/kg.
Serum osmolality (blood osmolality)
The blood will be very dilute, which means a decreased serum osmolality, under 270 mOsm/L.
Remember that a large portion of the substances in the blood is sodium — so when serum osmolality is low, sodium (Na) levels will also be low in a patient with SIADH. Sodium levels will be under 136 mEq/L, indicating hyponatremia.
Treatment (medications) for SIADH
One of the important medications for SIADH is a diuretic to try to eliminate the excess fluid. Within diuretics, you have loop diuretics (furosemide), thiazide diuretics (hydrochlorothiazide), osmotic diuretics (mannitol), and potassium sparing diuretics (spironolactone).
Want to learn about diuretics in more detail? These medications are covered in our Pharmacology Flashcards.
We can also give the patient a vasopressin antagonist. Remember that vasopressin is ADH and an antagonist blocks — blocking production of ADH makes sense for a patient with excess or uncontrolled ADH production.
A patient with SIADH and hyponatremia can also be given hypertonic saline, an IV sodium solution, to slowly raise the sodium levels in their body and allow their electrolytes to balance.
Nursing care for SIADH
For a patient with SIADH, you will monitor their intake & output and weigh daily, just like the patient with diabetes insipidus.
You will restrict fluids and replace sodium as ordered by the provider.
Monitor for fluid volume excess. If the patient has too much fluid volume overload, that can lead to pulmonary edema which is life threatening, so it’s important to monitor for that.
In the case of pulmonary edema, along with calling the healthcare provider, the nurse's priority action is to sit the patient up in tripod position over the bedside table.
Remember that hyponatremia can lead to confusion. So you will want to monitor the patient’s neurological status for that. Also, you may need to implement seizure precautions, because hyponatremia can lead to seizures if it becomes severe.
Diabetes insipidus vs. SIADH labs
|Lab Value||Diabetes insipidus||SIADH|
|Urine specific gravity||Low (urine is dilute)||High (urine is concentrated)|
|Urine osmolality||Low (urine is dilute)||High (urine is concentrated)|
|Serum osmolality||High (blood is concentrated)||Low (blood is dilute)|
|Sodium||High (hypernatremia)||Low (hyponatremia)|
Cathy’s teaching on these disorders is intended to help prepare you for Medical-Surgical nursing exams. The Medical-Surgical Nursing video series is intended to help RN and PN nursing students study for nursing school exams, including the ATI, HESI and NCLEX.
In this video we are going to talk about diabetes insipidus, as well as SIADH. These are two very important topics. If you happen to be following along with cards, I'm on card 23 [in the endocrine system section of the Medical-Surgical flashcards], and you'll notice the next four cards, there's a lot of bold and red text on them, because there are several very important nursing concepts for you to know related to these disorders.
So both of these disorders have to do with ADH, either too much ADH, not enough ADH, or the organs in your body aren't responding appropriately to ADH. So let's do a quick review of ADH, and if you want more details about ADH, I did make a whole other video about it. But if you recall, ADH is released by the posterior pituitary gland in response to low blood volume in the body, low blood pressure, or to hypernatremia, or increased blood osmolarity. So if the body senses any of these three things, it will release ADH from the posterior pituitary gland, which will cause the kidneys to reabsorb more water, which helps to bring up that blood pressure, bring up that blood volume, and dilute the blood so that the blood osmolarity is back to a normal level.
So with diabetes insipidus, we have one of two things going on.
We may have neurogenic diabetes insipidus, which means that there is some kind of injury or tumor in the hypothalamus or the pituitary gland such that insufficient ADH is being released from the posterior pituitary gland. So if the posterior pituitary gland is not releasing enough ADH, the kidneys aren't getting the signal to reabsorb that water. So that is neurogenic diabetes insipidus.
The other thing we may have going on is something called nephrogenic diabetes insipidus. So in this situation, the posterior pituitary gland is doing its job fine. It's releasing ADH. But there's some problem in the kidneys and they are not responding appropriately to that signal. Right? They're getting the ADH but they're kind of like, "Ehh." They're not reabsorbing more water. This may be due to some kind of kidney infection, or maybe the kidneys have been damaged due to nephrotoxic medications. But this is called nephrogenic diabetes insipidus.
So in terms of signs and symptoms of diabetes insipidus, the key symptom is that the patient will have large amounts of dilute urine. So we're just dumping water. We're just peeing out tons of dilute urine. We're not reabsorbing that water as we should be. The patient will exhibit polydipsia, so this is where they have excessive thirst. And one way that I remember this symptom, if you look at the word diabetes insipidus, diabetes inSIPidus will make you want to SIP more water, because you're super thirsty.
Other signs and symptoms include dehydration, hypotension, and anorexia.
So in terms of labs, labs are definitely going to be important to know.
The urine, like we talked about, is going to be very dilute. So the specific gravity of the urine will be very low, or under 1.005. Also the osmolarity of the urine will also be very low, because it's very dilute. So it will be under 200, and then we would expect kind of decreased sodium levels in the urine as well.
In the blood though, that's a totally different story. Because we're getting rid of all those fluids, the blood's going to be really concentrated, so the serum, or blood osmolarity is going to be over 300, and the sodium levels are also going to be really high, so we're going to see hypernatremia.
In terms of diagnosis of diabetes insipidus, we can do a water deprivation test to test the ability of the kidneys to concentrate urine. We can also do a vasopressin test. So vasopressin is essentially the same as ADH. And if we give the patient vasopressin, we would expect their kidneys to reabsorb water. If their kidneys don't do that, then we know it's a problem with the kidneys and we have nephrogenic diabetes insipidus. If we give the patient vasopressin and their kidneys do do their job and reabsorb water, then we know it was an issue with the pituitary gland not producing enough ADH.
And if that is the case, then we can provide medications such as vasopressin or desmopressin, which you can find on Pharm card 110 [in our Pharmacology Flashcards] in terms of getting more information about those medications.
So in terms of nursing care, when a patient has diabetes insipidus, we're definitely going to want to monitor the patient's I&Os, as well as their urine specific gravity and their daily weight.
Let's move on to syndrome of inappropriate ADH, or SIADH. With SIADH, we have excess release of ADH from the posterior pituitary gland due to a brain tumor, head injury, meningitis, or because of a medication. So even though the blood is very dilute, the posterior pituitary gland still releases ADH, even though it should not. So because it's releasing all this extra ADH inappropriately, it's causing the kidneys to reabsorb more water.
So signs and symptoms of SIADH will include a very small amount of very concentrated urine. And then we'll have signs and symptoms of fluid volume excess because we have all this extra fluid being reabsorbed. So the patient may exhibit signs and symptoms such as tachycardia, hypertension, crackles, jugular vein distention, as well as weight gain. They may also complain of a headache, weakness, and muscle cramping. And then they may also exhibit confusion because we have hyponatremia due to all this dilution, and that can cause the patient to be confused.
So one way to remember what SIADH does, if you look at the first two letters of that, S-I, you can think of super inflated, and that's basically what happens. When you have SIADH, you reabsorb all this extra fluid, and you are super inflated.
In terms of labs, we're going to have, basically, the exact opposite as we saw with diabetes insipidus.
So the urine with SIADH will be super concentrated. So the urine specific gravity will be elevated. It will be over 1.03, and the urine osmolality or osmolarity will also be increased because it's so concentrated.
The blood or the serum will be a completely different story. It will be very dilute, so the serum osmolality will be decreased. It will be under 270 and we'll see hyponatremia because the sodium is diluted with all this extra fluid volume.
In terms of treatment, we're definitely going to give the patient diuretics to try to get rid of some of this excess fluid. We can also give them a vasopressin antagonist, and then we can also give them hypertonic saline to help bring up the sodium levels and allow for those electrolyte levels to be more in balance.
In terms of nursing care, again, we're going to want to monitor the patient's I&Os, we're going to weigh our patients daily, and we're going to restrict fluids and replace sodium as ordered by the provider. We're going to monitor for fluid volume excess. So if we have too much fluid volume overload, we can end up with pulmonary edema, which is life threatening. So we're definitely going to want to monitor for that. And then we're going to want to continually monitor the patient's neurologic status because of the confusion that can result from hypernatremia, and we're also going to need to implement seizure precautions, because hypernatremia can result in seizures if it gets too bad.
So hopefully that was useful in terms of a review of diabetes insipidus, and SIADH. If you appreciated this review, be sure to like our video and subscribe to us here. Take care.
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