Diabetes Insipidus vs. Diabetes Mellitus: What Are the Differences?

Diabetes insipidus and diabetes mellitus are unrelated diseases with similar names. This connection dates back in history before researchers understood that although the conditions have similar symptoms, the causes are markedly different.

Diabetes insipidus (DI) is a rare condition in which the kidneys are unable to retain water, whereas diabetes mellitus is a condition characterized by the inability of the body to produce enough or respond normally to insulin, causing blood sugar (glucose) levels to be abnormally high.

This article will walk through the differences in each of these conditions, what the causes are, and how each is diagnosed and treated. 

Causes of Diabetes Insipidus vs. Diabetes Mellitus

Causes of Diabetes Insipidus

Vasopressin (VP), also known as antidiuretic hormone (ADH), is a hormone produced by the hypothalamus and stored in the pituitary gland that helps regulate the osmolarity (concentration) of fluids in the body.

ADH mainly regulates water levels by controlling urine production. When fluid levels are low in the body, ADH is released to conserve water. ADH increases the reabsorption of water in the kidneys. This decreases the volume of urine produced.

In diabetes insipidus, ADH fails to properly regulate the body's level of water and allows too much urine to be produced and passed from your body. This massive loss of water can lead to dehydration and electrolyte imbalances. 

DI occurs in one of two ways. DI that results from a lack of ADH production by the hypothalamus is called central DI. DI that results from nonworking vasopressin receptors in the kidneys is called nephrogenic DI. 

DI can be acquired or inherited. Common causes of acquired DI include:

• Head injury or trauma
• Surgery
• Brain tumors
• Chronic use of lithium (mood-stabilizing medication)

Less common causes of acquired DI include:

• High levels of calcium in the blood (hypercalcemia) 
• Low levels of potassium in the blood (hypokalemia) 
• Kidney infection (pyelonephritis)
• Ureteral obstruction, such as a kidney stone
• Infections of the brain
• Events that suddenly decrease oxygen to the brain, like a stroke

Head injury or trauma, surgery, and brain tumors commonly cause central DI, whereas long-term use of lithium and electrolyte imbalances are the most common cause of nephrogenic DI. 

Lastly, primary polydipsia (PPD), also known as dipsogenic DI, is caused by a defect of the thirst mechanism within the hypothalamus. Excessive thirst causes you to drink more fluids overriding the body’s capacity to conserve water. PPD is associated with mental disorders, but exactly why the two are often linked together remains a mystery.

Causes of Diabetes Mellitus

Diabetes mellitus comes in two forms: type 1 and type 2 diabetes. The cause of diabetes mellitus depends on the type you have.

Type 1 diabetes has multiple genetic predispositions and environmental factors that are still poorly defined and not associated with obesity.

Type 1 diabetes is also associated with other autoimmune diseases, including: 

• Graves' disease (autoimmune disorder associated with an overactive thyroid)
• Hashimoto's thyroiditis (autoimmune disorder associated with an underactive thyroid)
• Addison's disease (adrenal glands don't make enough of certain hormones)
• Vitiligo (condition causing patches of skin without pigment, or color)
• Celiac disease
• Autoimmune hepatitis (intolerance to gluten found in wheat, barley, and rye)
• Myasthenia gravis (autoimmune disorder that attacks your liver cells)
• Pernicious anemia (vitamin B12 deficiency causing a lack of red blood cells)

Symptoms of Diabetes Insipidus vs. Diabetes Mellitus

Diabetes Insipidus and Diabetes Mellitus Symptoms 

DI and DM have very different causes and symptoms, but the initial symptoms of exhaustion, increased thirst, excessive urination (polyuria), and blurred vision are symptoms that you can experience with both conditions. This underscores the importance of getting a full evaluation.

Symptoms Unique to Diabetes Insipidus

DI is defined as the passage of large volumes (more than 3 liters in a 24-hour period) of diluted urine (less than 300 milliosmoles per kilogram). A healthy adult typically passes less than 3 quarts a day, whereas someone with DI may produce as much as 5 to 10 times that amount.

Losing large volumes of fluid can make you feel tired and dehydrated. As a result, most people feel extreme thirst (polydipsia) and therefore drink more fluids to replace the water that they’ve lost.

Additional symptoms of diabetes insipidus include:

• Extremely high output of urine that has little-to-no color or odor
• Needing to get up at night to urinate
• Bedwetting
  

Symptoms Unique to Diabetes Mellitus

The symptoms of type 1 and type 2 diabetes mellitus are about the same. They result from elevated blood sugar levels. The most common symptoms are:

• Polyuria: excessive urination, often at night
• Polydipsia: excessive thirst or thirst that cannot be quenched
• Polyphagia: excessive hunger, often paired with weight loss
• Weight loss
• Numbness and tingling in the hands and feet
• Feeling very tired or fatigued
• Dry skin
• Sores that heal slowly
• Having more infections than usual

Diagnosing Diabetes Insipidus vs. Diabetes Mellitus

How Is Diabetes Insipidus Diagnosed?

Diagnosing DI involves lab tests and imaging studies meant to evaluate how the kidneys are functioning. Some of the tests that may be used during diagnosis are:

• Blood test for sodium levels: Measuring sodium levels in the blood is a useful method of helping to diagnose DI and may even determine the type of DI that you have. If serum sodium levels are high, your DI may be due to central or nephrogenic causes, but if your serum sodium levels are normal or low, your DI is likely due to primary polydipsia, also known as dipsogenic DI.
• Urinalysis: If DI is suspected, you may be asked to take a urinalysis, a urine test that involves checking the appearance, concentration, and content of your urine. A urinalysis is usually requested if you have a chief complaint of polyuria, or excessive urination. In addition to analyzing the urine’s concentration (osmolarity), you may be asked to take a 24-hour urine sample. 
• Water deprivation test: A healthcare professional can diagnose DI by assessing your body’s ability to retain water. A water deprivation test consists of restricting water for several hours and observing the type of urine that is produced. Under normal circumstances, the body will conserve water and produce a small amount of concentrated urine, but if you have DI you will produce a large amount of dilute urine.

Magnetic resonance imaging (MRI) may also be used to look for related issues. An MRI is mostly used to look at the brain and kidneys for tumors or anatomical variations in the hypothalamus or pituitary gland that may be the cause of your DI.

How Is Diabetes Mellitus Diagnosed?

If diabetes mellitus is suspected, a fasting blood glucose level may be taken, as this is often the quickest and cheapest way to confirm the condition. 

High blood glucose may prompt a healthcare professional to measure your blood sugar after an overnight fast, or a time in which you have not eaten for 8 hours or more.

• A fasting blood sugar level of 99 milligrams per deciliter (mg/dL) or lower is considered normal
• A blood sugar level of 100–125 mg/dL indicates that you have prediabetes
• Blood sugar that is 126 mg/dL or higher indicates that you have diabetes.

Blood sugar levels can rise rapidly if you are stressed or ill, so a fasting blood sugar test is usually repeated a few days later to confirm the diagnosis. 

A blood test measuring hemoglobin A1C, a three-month average of blood sugar, is the most common method used to get and confirm a diagnosis of diabetes mellitus, but it requires confirmation at a laboratory.

Treating Diabetes Insipidus vs. Diabetes Mellitus

Treatment for Diabetes Insipidus

Treatment for DI depend on what's causing the condition. Treatment for each condition include the following:

Central DI (CDI): If the cause of your CDI is due to a lack of ADH production taking a vasopressin analog, called desmopressin may relieve your symptoms. If a brain tumor or growth is the cause of your CDI, surgery may be indicated.

Nephrogenic DI (NDI): If your NDI is caused by a medication like lithium, discontinuing the drug will likely resolve your NDI. Only stop taking the drug under the guidance of a healthcare provider.  

NDI is usually caused by nonworking vasopressin receptors on the kidneys, so desmopressin will not help. In this case, taking nonsteroidal anti-inflammatory drugs (NSAIDs) or a thiazide diuretic and eating a low-salt diet may be helpful by lowering the amount of urine produced by the body.

Dipsogenic DI (DDI): Currently, there is no effective treatment for DDI. Sucking on ice chips may help reduce thirst and taking desmopressin before bedtime may help limit nighttime bed-wetting. 

Gestational DI: Gestational DI (as a result of pregnancy) usually goes away after the baby is born, but desmopressin may be used during pregnancy as the drug is safe for both the mother and baby. 

Treatment for Diabetes Mellitus

There is no cure for type 1 or type 2 diabetes, but a combination of oral medication, injectable insulin, and lifestyle changes can help you manage and prevent the progression of your diabetes.

Medication: Daily insulin via injection or an insulin pump is the mainstay of treatment for type 1 diabetes. Oral medications, such as Glucophage (metformin), that regulate blood sugar levels by changing the way the body makes or uses sugar is much more common in type 2 diabetes.

Insulin: Over time, a person with type 2 diabetes may need to add insulin to their treatment regimen if the combination of lifestyle modifications and oral medication does not control blood sugar levels.

Lifestyle changes:  In addition to taking your medication as prescribed, the best way to manage type 1 or type 2 diabetes is to monitor the amount of carbohydrates in your diet, lead an active and healthy lifestyle, and manage your blood pressure and cholesterol levels. 

If you have type 1 diabetes, it is particularly important to pay careful attention to your blood sugar levels, particularly when you are exercising. 

Summary

Despite having similar symptoms and names, diabetes insipidus (DI) and diabetes mellitus are completely unrelated diseases. DI is caused by kidney function problems, while diabetes mellitus is caused by dysregulation of blood sugar levels. Both conditions may be diagnosed using a combination of blood and urine tests. Treatments vary based on the cause and severity of each condition.

A Word From Verywell

Diabetes insipidus and diabetes mellitus have similar symptoms, but they are completely different conditions that require specific diagnostic tests and treatment. Diabetes mellitus is also much more common than DI.

Fortunately, both conditions are highly treatable. If you exhibit symptoms of either, seek immediate medical attention from a healthcare professional.