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:
| Test | Result | Reference Range |
|---|---|---|
| Thyroid-stimulating hormone (TSH) | 1.1 µU/mL | 0.4–4.0 µU/mL |
| Free thyroxine (T4) | 1.2 ng/dL | 0.9–1.7 ng/dL |
| Total triiodothyronine (T3) | 50 ng/dL | 80–200 ng/dL |
Which of the following is the most likely mechanism responsible for this patient's thyroid laboratory findings?
- AAutoimmune destruction of thyroid follicular cells
- BInfarction of the anterior pituitary gland
- Decreased peripheral conversion of thyroxine to triiodothyronineCevap
- DIncreased hepatic synthesis of thyroid-binding globulin
- ERelease of preformed thyroid hormone from the gland
Cevap
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.
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Euthyroid sick syndrome (non-thyroidal illness syndrome)