Euthyroid sick syndrome is low serum levels of thyroid hormones in clinically euthyroid patients with nonthyroidal systemic illness. Diagnosis is based on excluding hypothyroidism. Treatment is of the underlying illness; thyroid hormone replacement is not indicated.

Patients with various acute or chronic nonthyroid disorders may have abnormal thyroid function tests. Such disorders include acute and chronic illness, particularly fasting, starvation, protein-energy undernutrition, major trauma, MI, chronic renal failure, diabetic ketoacidosis, anorexia nervosa, cirrhosis, thermal injury, 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. Patients are clinically euthyroid and do not have elevated thyroid-stimulating hormone (TSH) levels.

Pathogenesis is unknown but may include decreased peripheral conversion of T₄ to T₃, decreased clearance of rT₃ generated from T₄, and decreased binding of thyroid hormones to thyroxine-binding globulin (TBG). Proinflammatory cytokines (eg, tumor necrosis factor-α, IL-1) may be responsible for some changes.

Interpretation of abnormal thyroid function test results in ill patients is complicated by the effects of various drugs, including the iodine-rich contrast agents and amiodarone, which impairs the peripheral conversion of T₄ to T₃, and by drugs such as dopamine and corticosteroids, which decrease pituitary secretion of TSH, resulting in low serum TSH levels and subsequent decreased T₄ secretion.

Pearls & Pitfalls Thyroid function tests should not be ordered for ICU patients unless thyroid dysfunction is highly suspected.

Diagnosis

• TSH
• Serum cortisol
• Clinical judgment

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 as it would be in hypothyroidism. Serum rT₃ is elevated, although this measurement is rarely done. 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 tests in the acutely or chronically ill patient. Unless thyroid dysfunction is highly suspected, thyroid function tests should not be ordered for patients in the ICU.

Treatment

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 TSH levels, unlike the marked TSH elevations present in true hypothyroidism.

Last full review/revision May 2012 by Jerome M. Hershman, MD