A 60-year-old man in the intensive care unit with severe sepsis has serum T3 of 40 ng/dL (low), free T4 at the lower limit of normal, reverse T3 slightly elevated, and TSH of 1.5 mIU/L (normal). Which mechanism best explains the low T3?

• A Impaired 5-prime-deiodination of T4 to T3 in peripheral tissues ✓
• B Failure of the pituitary to secrete TSH secondary to infarction
• C Increased clearance of T3 by the liver without altered production
• D Autoimmune destruction of thyrocytes lowering glandular output

Explanation

In non-thyroidal illness, cytokines and fasting reduce activity of type 1 and type 2 deiodinases, so less T4 is converted to active T3, producing the low T3 state with normal TSH. The modest rise in reverse T3 reflects shunting toward inactive metabolites. Pituitary failure would show an inappropriately low or undetectable TSH rather than a normal value, and autoimmune destruction would elevate TSH once the free T4 fell.

Reference: Williams Textbook of Endocrinology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.