# Situation: A 63-year-old man with small cell lung cancer is admitted with headache and mild confusion. His blood pressure is 128/80 mmHg, his mucous membranes are moist, and he has no edema. He takes sertraline for depression. His laboratory results are shown below. Serum sodium: 124 mEq/L (135–145) Serum potassium: 4.1 mEq/L (3.5–5.0) Serum osmolality: 258 mOsm/kg (275–295) Urine osmolality: 520 mOsm/kg Urine sodium: 48 mEq/L Thyroid function: normal Which condition do these findings MOST suggest?

> source: MyMerci (mymerci.kr)  
> url: https://mymerci.kr/pages/nclex_q.php?qn_id=630412  
> language: ko  
> subject: Nursing Practice IV — Care of Clients with Problems in Nutrition and Gastrointestinal, Metabolism and Endocrine, Perception and Coordination

## 문제

Situation: A 63-year-old man with small cell lung cancer is admitted with headache and mild confusion. His blood pressure is 128/80 mmHg, his mucous membranes are moist, and he has no edema. He takes sertraline for depression.

His laboratory results are shown below.

Serum sodium: 124 mEq/L (135–145)
Serum potassium: 4.1 mEq/L (3.5–5.0)
Serum osmolality: 258 mOsm/kg (275–295)
Urine osmolality: 520 mOsm/kg
Urine sodium: 48 mEq/L
Thyroid function: normal

Which condition do these findings MOST suggest?

## 보기

1. Arginine vasopressin deficiency (central diabetes insipidus)
2. Low-volume hyponatremia from poor oral intake
3. Syndrome of inappropriate antidiuretic hormone (SIADH) **✔ 정답**
4. Primary adrenal insufficiency (Addison disease)

**정답: 3**

## 해설

Low serum sodium and osmolality with inappropriately concentrated urine, urine sodium above 30 mEq/L, normal volume status, and normal thyroid function fit SIADH; small cell lung cancer and sertraline are both known causes. Primary adrenal insufficiency usually brings low blood pressure and high potassium, and low-volume hyponatremia produces dry mucosa and a low urine sodium.

## 심화 해설

The condition the findings suggest
He has small cell lung cancer and takes sertraline, and his laboratory results show serum sodium 124 mEq/L, serum osmolality 258 mOsm/kg, urine osmolality 520 mOsm/kg, and urine sodium 48 mEq/L, with normal thyroid function. Low serum sodium and osmolality with inappropriately concentrated urine, a urine sodium above 30 mEq/L, normal volume status, and normal thyroid function fit the syndrome of inappropriate antidiuretic hormone (SIADH). Small cell lung cancer and sertraline are both known causes.

The mechanism of SIADH
Antidiuretic hormone (ADH), also called arginine vasopressin, makes the collecting ducts reabsorb water. In SIADH, ADH is released despite low serum osmolality, either from tumor cells (small cell lung cancer can produce it) or because of drugs such as selective serotonin reuptake inhibitors. The kidneys retain free water, which dilutes the serum sodium. The urine is inappropriately concentrated for the low serum osmolality, and urine sodium stays high because the body is not volume-depleted. The client looks euvolemic: blood pressure is normal, mucous membranes are moist, and there is no edema. Headache and confusion reflect brain swelling from the water shift.

| Condition | Serum sodium | Urine | Other clues |
| --- | --- | --- | --- |
| SIADH | Low | Concentrated; urine sodium above 30 | Euvolemic |
| Vasopressin deficiency | High | Large volumes, dilute | Thirst, polyuria |
| Low-volume hyponatremia | Low | Urine sodium below about 30 | Dry mucosa, hypotension |
| Primary adrenal insufficiency | Low | Variable | Hypotension, high potassium |

Why the other conditions do not fit
Arginine vasopressin deficiency (central diabetes insipidus) produces large volumes of dilute urine and a high serum sodium. Low-volume hyponatremia would produce dry mucous membranes and a urine sodium below about 30 mEq/L as the kidneys conserve sodium. Primary adrenal insufficiency usually causes hypotension and hyperkalemia; his blood pressure and potassium are normal.

Nursing application
The nurse anticipates fluid restriction, close monitoring of sodium, neurologic status, intake and output, and daily weight, and seizure precautions if sodium falls further. The nurse reports worsening confusion promptly.

Exam takeaway
Key point SIADH: low serum sodium and osmolality, concentrated urine, high urine sodium, and normal volume status. Think small cell lung cancer and SSRIs as causes.

## 임상 시나리오

Recognizing SIADHHyponatremia with concentrated urine
Serum sodium 124 mEq/L and osmolality 258 mOsm/kg with urine osmolality 520 mOsm/kg and urine sodium 48 mEq/L in a euvolemic client indicate SIADH.

Excess antidiuretic hormone makes the kidneys retain free water, diluting sodium. Small cell lung cancer and sertraline are recognized causes.

CautionWorsening headache and confusion reflect brain swelling. Monitor neurologic status closely and use seizure precautions if sodium falls further.

## 핵심 개념

- **SIADH** — Syndrome of inappropriate antidiuretic hormone, in which excess ADH causes water retention and dilutional hyponatremia.
- **Antidiuretic hormone** — Arginine vasopressin, a pituitary hormone that increases water reabsorption in the kidney.
- **Euvolemia** — Normal body fluid volume, without signs of dehydration or edema.
- **Serum osmolality** — The concentration of dissolved particles in blood, mainly determined by sodium.

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