Does systemic inflammation reduce T3 availability and impair thyroid hormone signaling?

Systemic inflammation significantly alters thyroid hormone homeostasis, a state often referred to as Non-Thyroidal Illness Syndrome (NTIS) or Euthyroid Sick Syndrome. This process involves both a reduction in the production of active hormone and a decrease in the cellular ability to respond to it.

Impact on deiodinase activity and T3 availability

Inflammation directly suppresses the conversion of thyroxine (T4) into the biologically active triiodothyronine (T3).

• Enzymatic inhibition: Pro-inflammatory cytokines, specifically IL-6 and TNF-α, downregulate Type 1 (DIO1) and Type 2 (DIO2) deiodinases. Since 80% to 90% of circulating T3 is derived from peripheral conversion by these enzymes, their inhibition leads to a profound drop in systemic T3 levels.
• Active inactivation: Inflammation concurrently induces Type 3 deiodinase (DIO3), which actively converts T4 and T3 into inactive forms (reverse T3 and T2), further depleting the pool of active hormone.
• Clinical correlations: Research in critically ill populations demonstrates that low T3 levels correlate strongly with elevated inflammatory markers, including C-reactive protein (CRP) and IL-1β.

Reductions in receptor sensitivity

Beyond lowering hormone levels, systemic inflammation creates a state of "acquired thyroid hormone resistance" at the cellular level.

• Transcriptional interference: The activation of the NF-κB pathway leads to "transrepression," where inflammatory signals compete for essential nuclear coactivators (such as p300/CBP) required for thyroid hormone receptors (TRα and TRβ) to function.
• Epigenetic and physical changes: Chronic inflammation can lead to the physical downregulation of TR expression. For instance, inflammatory states can trigger DNA hypermethylation or the upregulation of microRNAs like miR-34a-5p, which silence receptor expression.
• Signaling failure: These mechanisms prevent thyroid hormones from activating genes responsible for metabolic regulation, even if hormone levels are supplemented.

Bottom line

Systemic inflammation reduces T3 availability by inhibiting DIO1/DIO2 enzymes and induces peripheral hormone resistance by impairing thyroid hormone receptor sensitivity through NF-κB-mediated interference. This dual mechanism ensures a rapid reduction in metabolic activity during periods of high physiological stress.