Can inflammation and systemic illness lower circulating T3 by reducing T4-to-T3 conversion?

Systemic illness and acute or chronic inflammation profoundly alter thyroid hormone homeostasis, establishing a state clinically recognized as non-thyroidal illness syndrome (NTIS).

Mechanistic pathways of deiodinase dysregulation

• Cytokine Signaling: Elevated pro-inflammatory cytokines, specifically interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-α), and interleukin-1 beta (IL-1β), suppress the transcription and catalytic activity of type 1 (D1) and type 2 (D2) deiodinase enzymes via NF-κB and JAK/STAT pathways.
• Oxidative Stress and Thiol Depletion: Inflammatory cascades trigger an oxidative burst, depleting intracellular glutathione and other thiol pools. Because D1 and D2 are selenoproteins that rely on reducing equivalents as cofactors, this depletion selectively halts their ability to convert thyroxine (T4) to active triiodothyronine (T3).
• Upregulation of Deiodinase 3: Concurrently, inflammation induces the expression of type 3 deiodinase (D3) in peripheral tissues, including the liver and skeletal muscle, which actively degrades thyroid hormones.

Impact on circulating thyroid hormones

• Impaired T3 Generation: Approximately 80% of circulating active T3 is derived from peripheral outer-ring deiodination of T4. Inhibiting hepatic and renal D1 and D2 activity directly impairs this systemic activation pathway.
• Accelerated Hormone Inactivation: The concurrent upregulation of D3 enhances inner-ring deiodination. This shifts peripheral metabolism to shunt T4 into inactive reverse T3 (rT3) and degrade existing T3, rapidly lowering circulating active T3 levels.

Bottom line

• Inflammation and systemic illness directly impair peripheral T4-to-T3 conversion by suppressing activating deiodinases (D1/D2) via cytokine-driven oxidative stress and thiol depletion, while upregulating the inactivating enzyme (D3), ultimately lowering circulating active T3.