Solution to "A 47-Year-Old Man With Empyema and Hyponatremia"

Robert M. Centor, MD

Disclosures

May 11, 2006

This is the solution to a case we presented recently. You may review the case here.

1. How do you evaluate his low serum sodium?

I have a standard method for evaluating serum sodium. I will present a stepwise system that I encourage you to use. Internal medicine requires some compulsivity -- and electrolytes disorders clearly require you to work through the possibilities carefully.

  1. Look for pseudohyponatremia. The easiest test here is a serum osmolality, as true hyponatremic patients have decreased serum osms.

    1. Causes of pseudohyponatremia include 3 endogenous and 2 exogenous factors:

      Endogenous

      Hyperglycemia

    2. Hypertriglyceridemia

    3. Paraproteinemia

  2. Exogenous (iatrogenic)

    IV mannitol

  3. Glycine used in transurethral prostatectomy

  4. If the patient has true hyponatremia, check volume status

    1. Volume contraction -- Volume contraction stimulates ADH, so the patient retains free water, making the patient susceptible to hyponatremia. Volume contraction-related hyponatremia responds to volume expansion.

    2. Edematous states -- This occurs mostly with heart failure and cirrhosis. These patients have effective intravascular volume depletion; again, ADH is stimulated, and they do not excrete free water. These conditions respond to treatment of the underlying cause.

    3. Euvolemic patients -- see below.

  5. The highest energy level with hyponatremia occurs when the patient is euvolemic.

    1. First, check urine osms. If the patient has dilute urine (urine osms < 100), then the problem is likely:

      1. Psychogenic polydipsia

      2. Beer drinker's potomania

      3. Tea and toast diet

    2. If the urine osms are elevated, first check for 4 conditions:

      1. Chronic kidney disease (CKD) -- elevated serum creatinine (probably 2 mg/dl or greater). CKD patients have an impaired ability to dilute their urine.

      2. Addison's disease

      3. Hypothyroidism

      4. Use of thiazide diuretics (which impair urinary dilution)

    3. If the urine osms are elevated and the above 4 conditions are excluded:

      1. Consider physiologic stimulants of ADH: nausea, vomiting, pain, narcotics

      2. Consider transient causes of increased ADH: pulmonary processes, intracranial processes

      3. Consider drugs; the most common class I have seen in 2006 is the SSRIs

      4. If all of these are absent, consider the possibility of a condition causing long-term SIADH

2. What are the likely causes of hyponatremia in this man?

The patient had a serum osm = 234 mOsm/kg, confirming true hyponatremia. Despite his mild edema, his exam was consistent with a euvolemic state.

When the patient was first admitted, we assumed that he had ADH release related to his empyema. However, we proceeded with the evaluation. First, we obtained a urine osm (332 mOsm/kg). This, in fact, is inappropriately high for his serum sodium. This result was consistent with our presumed diagnosis. However, like a runner who hits a home run, we touched all the bases and found:

TSH = 26.8 (normal range 0.6-4.7)

Free T4 < 0.023 (normal range 0.8-1.6)

Free T3 = 1.16 (normal range 2.3-4.2)

This confirmed a laboratory diagnosis of hypothyroidism. We went back to the bedside and discovered:

  • His energy level had decreased dramatically 4-5 years earlier;

  • His antecubital fossa revealed dry, coarse skin; and

  • As mentioned in the previous physical exam, his reflexes were sluggish.

The patient had a prolonged hospital course because of the empyema (he required more than 2 weeks of chest tube drainage). We started synthetic thyroid with dramatic clinical results. He expressed amazement at how much better he felt.

Two years later, this patient is doing well, euthyroid on daily synthetic thyroid supplementation.

  • Even though this patient most likely had SIADH secondary to his empyema, going through the evaluation compulsively led to an important diagnosis.

  • Because we focused all of our attention on his empyema, we missed the opportunity to make a clinical diagnosis of hypothyroidism. This happens often in medicine: We focus on the most life-threatening problem, and miss a second problem.

Read and participate in the discussion of this case here, and watch for another new case soon.

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