Sick Euthyroid State

Also known as nonthyroidal illness syndrome and euthyroid sick syndrome.

Pattern of ↓ serum T3 without a compensatory ↑ in TSH or clinically significant thyroid dysfunction, classically associated with critical illness.

Epidemiology and Risk Factors

Pathophysiology

Mechanisms for altered changes in thyroid hormone levels include:

Altered thyroid hormone levels may be physiologic adaptations to ↓ metabolic work in the setting of critical illness, and not pathophysiological.

• Altered deiodinase activity
  ↑ Hepatic and renal conversion of T₄ to rT₃, rather than T₃.
  • rT₃ is not clinically active and also acts as a functional antagonist of T₃
• ↑ Thyroid hormone binding to serum proteins
• ↓ TSH secretion
• ↓ TRH secretion
• ↓ T₄ transport into peripheral tissues

Aetiology

Clinical Features

Assessment

History

Exam

Investigations

Laboratory:

• TFT
  • ↓ T3
  • Normal, ↓, or ↑ T4
  • ↑ rT3
    Distinguishes from central hypothyroidism and sick euthyroid state.
  • ↓ T3:rT3
  • Normal, ↓, or (rarely) ↑ TSH
    
    • Typically normal in early acute illness
    • Falls in later illness
      Generally remains detectable; patients who fall to undetectable levels typically will be thyrotoxic following recovery of critical illness.

Diagnostic Approach and DDx

Management

• Manage underlying critical illness
• Correct malnutrition
• Give T3 if cardiogenic shock

Specific therapy:

• Pharmacological
  • T3 supplementation
    Evidence in:
    • Cardiac surgery patients
    • Hormonal resuscitation of potential heart donors
    • ↑ CO and functional capacity in cardiac failure

Supportive care:

• G
  • Feeding

Disposition:

• Recheck TFTs following recovery from critical illness

Marginal and Ineffective Therapies

Anaesthetic Considerations

Complications

Prognosis

Key Studies

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References

1. Farwell AP. Nonthyroidal illness syndrome. Current Opinion in Endocrinology, Diabetes & Obesity. 2013;20(5):478-484. doi:10.1097/01.med.0000433069.09294.e8