Can systemic inflammation alter peripheral thyroid-hormone metabolism?

Systemic illness commonly produces the non-thyroidal illness syndrome (NTIS) pattern—particularly low circulating T3—and inflammatory signaling is a meaningful contributor to this altered thyroid-hormone metabolism. The evidence supports impaired peripheral T4-to-T3 conversion more strongly than it supports increased reverse-T3 (rT3) production specifically.

Clinical evidence

• Low T3 is characteristic of NTIS. Reduced peripheral conversion of thyroxine (T4) to active triiodothyronine (T3) is a recognized contributor, alongside central hypothalamic-pituitary changes, altered hormone transport/metabolism, nutritional status, and illness severity.
• Acute febrile illness and cirrhosis have been associated with higher mean serum rT3 than in healthy controls, whereas this was not uniform in chronic renal failure. Across NTIS, rT3 may be high, normal, or low depending on the disease setting.
• In hospitalized COVID-19, inflammatory markers were inversely associated with T3 and the T3:T4 ratio, but not with rT3 or rT3:T3, illustrating that rT3 does not consistently track inflammation.

Mechanisms

• Type 1 deiodinase (DIO1), particularly in liver, converts T4 to T3. In human HepG2 hepatocyte-derived cells, IL-1β reduced DIO1 activity and dose-dependently inhibited the DIO1 promoter. TNF-α and IL-6 did not significantly reduce DIO1 enzyme activity under the tested conditions, indicating cytokine- and tissue-specific effects.
• Type 3 deiodinase (DIO3) inactivates T4 to rT3; increased tissue DIO3 has been reported in critical illness. Reduced DIO1 may also decrease rT3 clearance, so elevated rT3 cannot be assumed to reflect increased production alone.

Clinical implications

• Bottom line: Inflammation can suppress peripheral T4-to-T3 conversion, with moderate-confidence support. It can plausibly promote rT3 accumulation, but rT3 is variable and is not an established inflammation biomarker or guide for thyroid-hormone dosing.