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Zorluk: KolayEndocrinology and Metabolism

A 68-year-old man is evaluated in the intensive care unit on day three of a hospitalization for severe community-acquired pneumonia complicated by septic shock. He remains intubated and mechanically ventilated. His medical history is significant only for essential hypertension, and he has no prior history of thyroid disease. A comprehensive metabolic panel and thyroid function tests are ordered as part of a general evaluation. Laboratory results reveal a normal serum thyroid-stimulating hormone (TSH) level, a normal serum free T4T_4 level, and a significantly decreased serum total T3T_3 level. Which of the following is the most likely mechanism responsible for this patient's thyroid laboratory findings?

  1. Decreased peripheral conversion of T4T_4 to T3T_3Cevap
  2. B
    Impaired thyroid hormone synthesis at the follicular level
  3. C
    Inadequate stimulation by hypothalamic releasing factors
  4. D
    Unregulated autonomous production of thyroid hormone
  5. E
    Reduced hepatic production of thyroid carrier proteins

Cevap

Decreased peripheral conversion of T4T_4 to T3T_3
The correct answer identifies the pathophysiology of euthyroid sick syndrome (also known as non-thyroidal illness syndrome). In the setting of severe acute illness, such as sepsis or severe trauma, there is a marked decrease in the activity of 5′5'-deiodinase in peripheral tissues. This enzyme is normally responsible for the peripheral conversion of T4T_4 to the more biologically active T3T_3. Consequently, serum T3T_3 levels fall rapidly. In the early stages of this syndrome, TSH and free T4T_4 characteristically remain within the normal range. This is considered a physiologic adaptation to reduce metabolic rate and preserve energy during severe physiological stress. Supplementation with thyroid hormone is not indicated.

Adım Adım Çözüm

1
Analyze the patient's clinical setting.
The patient is critically ill in the intensive care unit with septic shock.
Severe physiological stress alters normal endocrine function, prompting consideration of stress-induced metabolic states.
2
Interpret the thyroid laboratory panel.
The patient has normal TSH, normal free T4T_4, and low total T3T_3.
This specific pattern isolates the abnormality to T3T_3 regulation rather than a primary pituitary or thyroid gland failure.
3
Correlate the lab findings with the clinical setting to identify the pathophysiology.
The combination of severe acute illness and an isolated low T3T_3 indicates euthyroid sick syndrome, caused by decreased peripheral 5′5'-deiodinase activity.
In critical illness, the body suppresses the conversion of T4T_4 to the highly active T3T_3 to conserve energy.

Anahtar Kavram

Euthyroid sick syndrome (non-thyroidal illness syndrome)
Tahmini Süre:50s
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Konular

Endocrinology and Metabolism

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