Does Thyroid-Stimulating Hormone (TSH) elevate during serious acute illness?

TSH Suppression During Serious Acute Illness

TSH is frequently suppressed during phases of acute illness, not elevated. 1

Thyroid Function Changes in Acute Illness

• Serum TSH is frequently suppressed during phases of acute illness, as noted by the U.S. Preventive Services Task Force in their recommendation statement on screening for thyroid dysfunction 1
• This pattern is part of what is known as the "nonthyroidal illness syndrome," "low T3 syndrome," or "euthyroid sick syndrome" 2, 3
• The typical pattern in acute illness includes low circulating T3, increased reverse T3, and when illness is severe and prolonged, low pulsatile TSH secretion and low circulating T4 levels 2

Biphasic Response in Acute Injury

• Some research suggests a potential biphasic response to injury, with a very short initial phase (1-2 hours) where TSH and T3 may be transiently elevated 4
• However, this quickly transitions to the more common pattern of suppressed thyroid function, which is fully established 6-18 hours after injury 4
• The second phase is characterized by reductions in serum TSH, T3, and total and free T4, with a rise in reverse T3 4

Prevalence and Clinical Significance

• The nonthyroidal illness syndrome may affect 60-70% of critically ill patients 3
• These changes can be seen within the first hours of critical illness and correlate with final outcome 3
• The severity of illness correlates with the degree of decrease in T3 (r=0.58) and T4 (r=0.38) 5
• Low T3 and T4 with low or undetectable TSH are associated with increased mortality 5

Mechanisms of TSH Suppression in Acute Illness

• Prolonged critical illness is associated with neuroendocrine dysfunction characterized by suppressed hypothalamic thyrotropin-releasing hormone (TRH) expression 2
• This results in reduced stimulation of the thyrotropes, whereby thyroidal hormone release is impaired 2
• Other factors affecting thyroid function during illness include:
  • Changes in thyroid hormone binding 2
  • Alterations in thyroid hormone uptake by cells 2
  • Changes in the activity of deiodinase enzymes 2
  • Medications such as dopamine, glucocorticoids, and other drugs 1

Important Clinical Considerations

• TSH secretion can vary by as much as 50% of mean values on a day-to-day basis, with up to 40% variation in values obtained from serial TSH measurements performed at the same time of day 1, 6
• This confirms the importance of not relying on a single TSH value to establish a diagnosis of thyroid dysfunction 1
• Serial TSH measurements are essential to establish that a thyroid disorder is real and persistent 1
• When interpreting thyroid function tests in critically ill patients, it's important to determine whether abnormal hormone measurements reflect an appropriate homeostatic response to severe illness or an independent metabolic disorder 7

Recovery Pattern

• Thyroid function generally returns to normal as the acute illness resolves 3
• However, some patients may continue to show alterations in thyroid function even after 6 months of therapy 5

In conclusion, serious acute illness typically causes TSH suppression rather than elevation, as part of a broader pattern of altered thyroid hormone metabolism that helps the body adapt to critical illness.