A 68-year-old man admitted with severe pneumonia has TSH 1.1 mIU/L, free T4 0.9 ng/dL, and total T3 55 ng/dL (normal 80 to 200). He is clinically euthyroid. Repeat testing 6 weeks after recovery shows a normal T3. What explains the initial pattern?

• A Central hypothyroidism secondary to pituitary suppression
• B Increased clearance of T4 by hepatic enzyme induction
• C Autoimmune destruction of the thyroid gland
• D Decreased peripheral conversion of T4 to T3 mediated by cytokines and cortisol ✓

Explanation

Non-thyroidal illness syndrome (euthyroid sick syndrome) produces low T3 from reduced 5'-deiodination of T4 driven by inflammatory cytokines, cortisol and illness severity; TSH and free T4 remain normal in mild cases. The normalisation after recovery without treatment confirms it is adaptive, not true thyroid disease. Treating with levothyroxine is not indicated. Autoimmune destruction would raise TSH eventually, and central hypothyroidism would show a low free T4 with inappropriately low TSH that persists.

Reference: Williams Textbook of Endocrinology, 14th ed.

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