Euthyroid sick syndrome is a condition in which serum levels of thyroid hormones are low in patients who have nonthyroidal systemic illness but who are actually euthyroid. Diagnosis is based on excluding hypothyroidism. Treatment is directed toward the underlying illness; thyroid hormone replacement is not indicated.
(See also Overview of Thyroid Function.)
Patients with various acute or chronic nonthyroid disorders may have abnormal thyroid function test results. Such disorders include fasting, starvation, protein-energy undernutrition, severe trauma, myocardial infarction, chronic kidney disease, diabetic ketoacidosis, anorexia nervosa, cirrhosis, thermal injury, drug overdose, and sepsis.
Decreased triiodothyronine (T3) levels are most common. Patients with more severe or prolonged illness also have decreased thyroxine (T4) levels. Serum reverse T3 (rT3) is increased. Thyroid-stimulating hormone (TSH) levels may be normal or even low, and during the recovery phase of the syndrome TSH levels may be slightly elevated.
Patients are actually euthyroid, but depending upon the underlying acute or chronic condition, may have features that overlap with severe hypothyroidism such as hypothermia, hypoventilation, hypotension, lethargy, or coma.
Pathogenesis is unknown but may include
Decreased peripheral conversion of T4 to T3
Decreased clearance of rT3 generated from T4
Decreased binding of thyroid hormones to thyroxine-binding globulin (TBG)
Proinflammatory cytokines (eg, tumor necrosis factor-alpha, interleukin-1) may be responsible for some changes.
Interpretation of abnormal thyroid function test results in ill patients is complicated by the effects of various medications, including the iodine-rich contrast agents and amiodarone, which impair the peripheral conversion of T4 to T3, and by medications such as dopamine and corticosteroids, which decrease pituitary secretion of TSH, resulting in low serum TSH levels and subsequent decreased T4 secretion.
Pearls & Pitfalls
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Diagnosis of Euthyroid Sick Syndrome
TSH
Serum cortisol
The diagnostic dilemma is whether the patient has hypothyroidism or euthyroid sick syndrome. The best test is measurement of TSH, which in euthyroid sick syndrome is low, normal, or slightly elevated but not as high (usually < 10 mIU/L [10 microIU/mL]) as it would be in primary hypothyroidism (see table Results of Thyroid Function Tests in Various Clinical Situations).
Serum rT3 is elevated, although this measurement is rarely done, and the results do not usually return in a time frame to be useful in decision making.
Serum cortisol is often elevated in euthyroid sick syndrome and low or low-normal in hypothyroidism due to pituitary-hypothalamic disease.
Because tests are nonspecific, clinical judgment is required to interpret abnormal thyroid function test results in patients who are acutely or chronically ill. Unless thyroid dysfunction is highly suspected, thyroid function tests should not be ordered in these patients.
Treatment of Euthyroid Sick Syndrome
Treatment of underlying disorder
Treatment with thyroid hormone replacement is not appropriate. When the underlying disorder is treated, results of thyroid tests normalize.
Key Points
Many seriously ill patients have low levels of thyroid hormones but are not clinically hypothyroid and do not require thyroid hormone supplementation.
Patients with euthyroid sick syndrome have low, normal, or only slightly elevated levels of thyroid-stimulating hormone (TSH), unlike the marked TSH elevations present in true hypothyroidism.
