A 65-year-old man is evaluated in the intensive care unit. He was admitted 5 days ago for … — USMLE Step 3

A $65$ -year-old man is evaluated in the intensive care unit. He was admitted $5$ days ago for severe community-acquired pneumonia complicated by septic shock. He remains on mechanical ventilation and requires continuous intravenous norepinephrine to maintain an adequate mean arterial pressure. His past medical history is significant for hypertension and type 2 diabetes mellitus, with no known history of thyroid disease. His current medications include broad-spectrum intravenous antibiotics, norepinephrine, and prophylactic subcutaneous heparin. Vital signs are temperature $37.2^\circ C$ ( $99.0^\circ F$ ), blood pressure $102/64$ mm Hg, pulse $98$ /min, and respirations $16$ /min (ventilator-controlled). Routine laboratory tests prompt the medical team to order a comprehensive thyroid panel, which yields the following results:

Laboratory Test	Result	Reference Range
Thyroid-stimulating hormone (TSH)	$0.2 \mu U/mL$	$0.4-4.0 \mu U/mL$
Total thyroxine (T4)	$4.1 \mu g/dL$	$5.0-12.0 \mu g/dL$
Free thyroxine (Free T4)	$0.7 ng/dL$	$0.9-1.7 ng/dL$
Total triiodothyronine (T3)	$55 ng/dL$	$80-200 ng/dL$

Which of the following is the most likely explanation for these laboratory findings?

Decreased peripheral conversion of thyroxine to triiodothyronineCevap
B
Inadequate secretion of thyrotropin from the anterior pituitary gland
C
Autoimmune destruction of thyroid follicular cells
D
Thyrotropin receptor stimulating antibodies
E
Viral-induced granulomatous inflammation of the thyroid

Cevap

Decreased peripheral conversion of thyroxine to triiodothyronine

This patient's laboratory findings (low TSH, low T4, and low T3) in the context of severe critical illness (septic shock) are characteristic of nonthyroidal illness syndrome (NTIS), also known as euthyroid sick syndrome. The primary pathophysiology involves decreased activity of

5'

-deiodinase, the enzyme responsible for the peripheral conversion of thyroxine (T4) to the biologically active triiodothyronine (T3). Consequently, T4 is preferentially converted to the inactive metabolite reverse T3 (rT3). In early or mild illness, TSH and T4 typically remain normal while T3 falls. In severe, prolonged illness (as seen in this ICU patient on vasopressors), central suppression of the hypothalamic-pituitary-thyroid axis also occurs, leading to decreased TSH and subsequently decreased free and total T4. This state is thought to be an adaptive mechanism to limit metabolic expenditure during critical illness. Treatment is supportive; exogenous thyroid hormone replacement is generally not indicated and may be detrimental.

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Evaluate the patient's clinical status and context.

The patient is critically ill in the ICU with septic shock and requiring life support (mechanical ventilation, vasopressors).

Establishing the severity of illness provides a critical pretest probability for stress-induced physiological changes rather than primary endocrine pathology.

Analyze the multi-parameter thyroid panel.

The panel shows low TSH, low total and free T4, and markedly low total T3.

Identifying the specific constellation of abnormal lab values dictates the differential diagnosis.

Correlate laboratory findings with the clinical context to determine etiology.

The simultaneous depression of TSH, T4, and T3 in a critically ill patient is classic for nonthyroidal illness syndrome (euthyroid sick syndrome).

Severe critical illness profoundly alters peripheral thyroid hormone metabolism and eventually suppresses the hypothalamic-pituitary-thyroid axis, preventing unnecessary metabolic expenditure.

Anahtar Kavram

Nonthyroidal illness syndrome (euthyroid sick syndrome) typically presents in critically ill patients with a low T3 level followed by low T4 and TSH levels due to decreased peripheral conversion and central axis suppression.

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