Snapshot A 65-year-old man with hypertension, hyperlipidemia, and diabetes presented to the emergency room for fevers, fatigue, and a productive cough over the past few days. His temperature is 102.1°F (38.9°C) and mean arterial pressure is 63 mmHg. Further evaluation demonstrates a leukocytosis, elevated procalcitonin, and a radiographic focal consolidation in his left lower lobe. He denies any temperature intolerance, weight changes, any nail and skin abnormalities, diarrhea, or constipation. He was admitted to the intensive care unit for treatment of sepsis secondary to pneumonia. On admission, labs for thyroid function were ordered and found to be abnormal. Introduction Clinical definition abnormal thyroid function tests in a normally functioning thyroid that occurs in the setting of a nonthyroidal illness Epidemiology Demographics frequently in hospitalized patients Risk factors acute illness sepsis diabetic ketoacidosis chronic illness cancer cardiac illness anorexia nervosa and fasting human immunodeficiency virus inflammatory bowel disease and other autoimmune diseases liver disease renal disease ETIOLOGY Pathogenesis hypothesized that reduced thyroid hormones may limit catabolism during illness cytokines downregulate thyroid hormone synthesis altered deiodinase enzyme activity ↓ conversion of thyroxine (T4) to (triiodothyronine) T3 ↑ conversion of T4 to reverse T3 ↓ thyroid binding globulin Presentation Symptoms and physical exam are variable and specific to underlying nonthyroidal illness Studies Diagnostic testing diagnostic approach evaluate lab findings in the context of acute or chronic illness studies thyroid function tests including TSH, total T4, free T4, total T3, free T3, and reverse T3 biopsy not indicated but would demonstrate a normal thyroid gland Differential Thyroid disease distinguishing factors physical exam and symptoms consistent with hyperthyroidism or hypothyroidism co-existing euthyroid sick syndrome can mask true thyroid disease Central hypopituitarism distinguishing factor loss of other hormones in addition to ↓ T4 include ↓ cortisol, sex steroids, growth hormone, and antidiuretic hormone Medication-related distinguishing factors perform medication review common medications include lithium, amiodarone, glucocorticoids, estrogens, androgens, and biotin supplementation DIAGNOSIS Diagnostic criteria typical findings ↓ T3 normal or ↑ T4 and reverse T3 normal or ↓ TSH severe findings ↓ T3, T4, and TSH Treatment Management approach treat and manage underlying illness thyroid hormone replacement is not indicated and can lead to overtreatment though controversial monitor thyroid function studies after the resolution of the acute illness episode Complications None