(See also Overview of Thyroid Function.)
Patients with various acute or chronic nonthyroid disorders may have abnormal thyroid function test results. Such disorders include acute and chronic illness, particularly fasting, starvation, protein-energy undernutrition, severe trauma, myocardial infarction, chronic kidney disease, 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-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 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.
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 test results in acutely or chronically ill patients. Unless thyroid dysfunction is highly suspected, thyroid function tests should not be ordered in these patients.
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.