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Difficulty: MediumEndocrinology and Metabolism

A 62-year-old man is evaluated in the intensive care unit on day 4 of an admission for severe urosepsis. He initially required mechanical ventilation and vasopressor support but is now hemodynamically stable and was extubated yesterday. He has a history of hypertension and benign prostatic hyperplasia. His medications include amlodipine, tamsulosin, and intravenous antibiotics. He has no history of thyroid disease. His temperature is 37.2°C (99.0°F), blood pressure is 118/76 mm Hg, and pulse is 82/min. Physical examination reveals clear lungs and a normal-sized, non-tender thyroid gland.

Laboratory studies show:

TestResultReference Range
Thyroid-stimulating hormone (TSH)1.1 µU/mL0.4–4.0 µU/mL
Free thyroxine (T4)1.2 ng/dL0.9–1.7 ng/dL
Total triiodothyronine (T3)50 ng/dL80–200 ng/dL

Which of the following is the most likely mechanism responsible for this patient's thyroid laboratory findings?

  1. A
    Autoimmune destruction of thyroid follicular cells
  2. B
    Infarction of the anterior pituitary gland
  3. Decreased peripheral conversion of thyroxine to triiodothyronineAnswer
  4. D
    Increased hepatic synthesis of thyroid-binding globulin
  5. E
    Release of preformed thyroid hormone from the gland

Answer

Decreased peripheral conversion of thyroxine to triiodothyronine (Euthyroid sick syndrome)
This patient's clinical picture of a severe systemic illness (urosepsis requiring ICU admission) combined with a low total T3, normal free T4, and normal TSH is classic for euthyroid sick syndrome, also known as non-thyroidal illness syndrome. In the setting of severe physiologic stress (such as sepsis, trauma, or prolonged starvation), the body decreases the activity of 5'-deiodinase, the enzyme responsible for the peripheral conversion of T4 (thyroxine) to the more active T3 (triiodothyronine). Concurrently, 5-deiodinase activity often increases, shunting T4 to the inactive metabolite reverse T3 (rT3). This is thought to be an adaptive mechanism to reduce metabolic rate and conserve energy during acute, severe illness. Thyroid hormone replacement is not indicated, and laboratory values typically normalize once the underlying illness resolves.

Step-by-Step Solution

1
Evaluate the patient's clinical context.
The patient is recovering from a severe systemic illness (urosepsis requiring ICU admission).
Severe physiologic stress alters normal metabolic pathways, including thyroid hormone metabolism.
2
Interpret the thyroid function tests.
The patient has a normal TSH, normal free T4, and a selectively low total T3.
Identifying the specific pattern of thyroid laboratory abnormalities is required to determine the underlying etiology.
3
Synthesize the clinical history with the laboratory findings to identify the diagnosis and its mechanism.
The pattern of low T3 with normal T4 and TSH in an ICU patient confirms euthyroid sick syndrome (non-thyroidal illness syndrome), driven by decreased 5'-deiodinase activity.
During severe illness, the body adapts by reducing the conversion of prohormone (T4) to active hormone (T3) to conserve energy.

Key Concept

Euthyroid sick syndrome (non-thyroidal illness syndrome)
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Topics

Endocrinology and Metabolism

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