SIADH vs. Diabetes Insipidus

Introduction to SIADH and Diabetes Insipidus

This article covers some important differences between two endocrine disorders: syndrome of inappropriate antidiuretic hormone (SIADH) and diabetes insipidus. 

Both SIADH and diabetes insipidus are disorders of the posterior pituitary.

For each of these disorders, this article will discuss the:

• Pathophysiology
• Etiology
• Most notable signs and symptoms
• Nursing care

SIADH

SIADH is an endocrine disorder characterized by fluid retention. The pathophysiology, etiology, signs and symptoms, and nursing care are described in the sections below.

Pathophysiology

SIADH is a disorder where the body is unable to suppress the secretion of ADH. The pituitary gland normally produces ADH. 

ADH tells the kidneys to retain water. If there is too much ADH being produced, such as in SIADH, this leads to reduced urine output and subsequent water retention. 

Antidiuretic hormone tells the kidneys to conserve and hold onto water.

Water retention causes hyponatremia. One way to remember the pathophysiology of SIADH is by thinking of the word “soggy.” There is too much water in the body.

In some cases of SIADH, an overproduction of ADH is caused by a condition affecting the pituitary gland. This could be something like a tumor in the pituitary gland.

The pituitary gland produces antidiuretic hormone (ADH).

Read: Addison Disease vs. Cushing Disease

Etiology

Etiologies of SIADH include:

• Oat cell carcinoma (a lung cancer). This is the most common etiology. The carcinoma causes the posterior pituitary to overproduce ADH.
• Viral pneumonia
• Head problems. Head problems include things like head trauma, neurosurgery, and tumors.

Related Video – What are SIADH and DI?

Signs and Symptoms

The notable signs and symptoms of SIADH include:

• Excess fluid volume. When the body retains too much fluid, the patient will present with hemodilution. Their serum sodium will also appear very low because it is diluted.
• Edema. Due to the excess volume, fluid will build up and cause edema. The ankles and legs are common places where edema forms.
• Hypertension. Because of the excess of fluid and reduced urine output, patients become hypertensive.
• Bounding pulses. The patient will have bounding pulses. The pulse will feel strong when the provider uses a finger to feel it. Note that this does not have anything to do with the heart rate.

Nursing Care

The nursing staff will monitor vital signs for complications. The vital signs they will monitor include:

• Heart rate
• Blood pressure
• Inputs and outputs, particularly with fluid volume overload
• Daily weight
• Assessing for pulmonary edema, which may develop because of the fluid volume overload.
• Placing the patient on seizure precautions. Sodium plays an important role in nerve transmission. As sodium levels go down in SIADH, the patient’s risk increases for seizures.

Diabetes Insipidus

Diabetes insipidus and SIADH are opposites. The unique thing about the endocrine system is that disorders occur in pairs. Once one is understood, so is its opposite.

Pathophysiology

Diabetes insipidus is an endocrine disorder where the kidneys pass an abnormally large volume of dilute, colorless urine. Diabetes insipidus is marked by frequent urination and intense thirst.

Of note, diabetes insipidus is a separate condition from diabetes mellitus. However, increased urination may be observed in both conditions, hence the term “diabetes,” which means urination. In fact, “diabetes insipidus” means “colorless urination,” while “diabetes mellitus” means “sweet urination.”

There are two types of diabetes insipidus:

1. Central diabetes insipidus, also called neurogenic diabetes insipidus. In central diabetes insipidus, there is a deficiency of ADH from the posterior pituitary due to damage to the pituitary. When the body does not have enough ADH to tell the kidneys to retain water, the result is the kidneys excreting too much water.
2. Nephrogenic diabetes insipidus. In nephrogenic diabetes insipidus, ADH is available, but the kidneys are not able to respond to it. Therefore, the kidneys can’t retain water and high volumes of diluted urine are excreted.

Etiology

The primary etiology of diabetes insipidus is head problems. That includes patients who have brain injuries, head trauma, neurosurgery, and brain tumors.

Related Video – What is Diabetic Ketoacidosis (DKA)?

Signs and Symptoms

The notable signs and symptoms of diabetes insipidus are:

• Frequent urination. Up to 20 liters of fluid may be lost with diabetes insipidus.
• Fluid volume deficit.
• Hemoconcentration. The sodium may appear falsely elevated.
• Hypotension.
• Compensatory tachycardia. The patient’s pulses will be weak and thready.
• Dry mucous membranes.
• Pale skin. 
• Weight loss.
• Hypovolemic shock. The biggest concern with diabetes insipidus is hypovolemic shock. Be cognizant of early signs of shock, such as low blood pressure that does not respond well to fluids.

A common symptom of diabetes insipidus is frequent urination.

Nursing Care

The nursing care is the same for diabetes insipidus as it is for SIADH. In diabetes insipidus, the medical team is concerned with fluid volume deficit, while in SIADH they are concerned with fluid volume excess. Therefore, vital signs are monitored in both disorders. Vital signs include:

• Heart rate
• Blood pressure
• Inputs and outputs, particularly fluid loss through urination
• Daily weights

For central diabetes insipidus, the provider may administer a synthetic antidiuretic hormone called desmopressin. For nephrogenic diabetes insipidus, the provider may be administering diuretics, such as hydrochlorothiazide.

Both SIADH and diabetes insipidus are disorders of the posterior pituitary, though they present as opposites. SIADH is characterized by fluid volume excess, while diabetes insipidus is characterized by fluid volume deficit and frequent urination. The provider must understand the disorder etiologies and be aware of the signs and symptoms of each disorder to provide appropriate care.