Chemistry, toxicology & urinalysis
Organ specific
Nonthyroidal illness affecting thyroid function tests

Topic Completed: 1 March 2011

Minor changes: 9 April 2020

Copyright: 2002-2020,, Inc.

Nat Pernick, M.D.
Page views in 2019: 35
Page views in 2020 to date: 53
Cite this page: Pernick N. Nonthyroidal illness affecting thyroid function tests. website. Accessed August 12th, 2020.
Definition / general
  • Also called euthyroid sick syndrome or low T4 syndrome
  • Seriously ill patients (sepsis, myocardial infarction, Angiology 2002;53:699) may have abnormal thyroid function tests without thyroid pathology
  • Due to reduced serum iodothyronine deiodinases (J Clin Endocrinol Metab 2003;88:3202)
  • Often caused by malnourishment (JPEN J Parenter Enteral Nutr 2006;30:415)
  • Initially these patients have low total T3 and free T3
  • Later thyroid binding globulin is disrupted causing low total T4
  • TSH is initially low to low-normal, then rises to normal or above normal as illness resolves, then returns to normal levels
  • TSH is suppressed by glucagon, dopamine and high doses of corticosteroids, which complicates interpretation
  • Common in medical intensive care unit (44%), and associated with poorer prognosis (Metabolism 2007;56:239)
Clinical images

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  • In critical illness, inhibition of 5' deiodinase causes reduced T3 and increased reverse T3 levels
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