03. Non-Thyroidal Illness Syndrome (Sick Euthyroid Syndrome)

Definition 

Illness-induced changes to thyroid hormone metabolism in patients with no intrinsic thyroid disease that include low T3, normal or low T4, and normal, low, or high levels of TSH.

Etiology

  • Mild illness: decreased conversion of T4 to T3 peripherally, leading to decreased T3 and increased reverse T3 (rT3), an inactive metabolite.
  • Severe illness: as above and decreased thyroid binding protein concentrations. Can also see transient secondary hypothyroidism from pituitary dysfunction.
  • TSH recovers first and may be significantly elevated during recovery phase.

Key Points 

  • Most common in critically ill patients.
  • Thyroid function should not be assessed in seriously ill patients unless there is a strong suspicion of thyroid dysfunction.
  • No treatment necessary. 
  • Follow-up testing needed as outpatient after illness recovery.

Bello G, Ceaichisciuci I, Silva S, et al. The role of thyroid dysfunction in the critically ill: a review of the literature. Minerva Anestesiol 2010. 11:919-928.

Langton JE, Brent GA. Nonthyroidal illness syndrome: evaluation of thyroid function in sick patients. Endocrinol Metab Clin North Am. 2002. 1:159-172.