(See also Overview of Thyroid Function Overview of Thyroid Function The thyroid gland, located in the anterior neck just below the cricoid cartilage, consists of 2 lobes connected by an isthmus. Follicular cells in the gland produce the 2 main thyroid hormones... read more .)
Patients with various acute or chronic nonthyroid disorders may have abnormal thyroid function test results Laboratory Testing of Thyroid Function The thyroid gland, located in the anterior neck just below the cricoid cartilage, consists of 2 lobes connected by an isthmus. Follicular cells in the gland produce the 2 main thyroid hormones... read more . Such disorders include acute and chronic illness, particularly fasting, starvation, protein-energy undernutrition Protein-Energy Undernutrition (PEU) Protein-energy undernutrition (PEU), previously called protein-energy malnutrition, is an energy deficit due to deficiency of all macronutrients. It commonly includes deficiencies of many micronutrients... read more , severe trauma, myocardial infarction Overview of Acute Coronary Syndromes (ACS) Acute coronary syndromes result from acute obstruction of a coronary artery. Consequences depend on degree and location of obstruction and range from unstable angina to non–ST-segment elevation... read more , chronic kidney disease Chronic Kidney Disease Chronic kidney disease (CKD) is long-standing, progressive deterioration of renal function. Symptoms develop slowly and in advanced stages include anorexia, nausea, vomiting, stomatitis, dysgeusia... read more , diabetic ketoacidosis Diabetic Ketoacidosis (DKA) Diabetic ketoacidosis (DKA) is an acute metabolic complication of diabetes characterized by hyperglycemia, hyperketonemia, and metabolic acidosis. Hyperglycemia causes an osmotic diuresis with... read more , anorexia nervosa Anorexia Nervosa Anorexia nervosa is characterized by a relentless pursuit of thinness, a morbid fear of obesity, a distorted body image, and restriction of intake relative to requirements, leading to a significantly... read more , cirrhosis Cirrhosis Cirrhosis is a late stage of hepatic fibrosis that has resulted in widespread distortion of normal hepatic architecture. Cirrhosis is characterized by regenerative nodules surrounded by dense... read more , thermal injury, and sepsis Sepsis and Septic Shock Sepsis is a clinical syndrome of life-threatening organ dysfunction caused by a dysregulated response to infection. In septic shock, there is critical reduction in tissue perfusion; acute failure... read more .
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 Hypothyroidism Hypothyroidism is thyroid hormone deficiency. It is diagnosed by clinical features such as a typical facial appearance, hoarse slow speech, and dry skin and by low levels of thyroid hormones... read more 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.
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