Can systemic inflammation suppress thyroid hormone activation and signaling?

Systemic inflammation significantly alters thyroid hormone homeostasis, driving a clinical state often recognized as non-thyroidal illness syndrome (NTIS).

Clinical and physiological impact

• Altered Hormone Profiles: Elevated levels of interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-α) directly lower active T3 production while raising inactive reverse T3 (rT3) levels. This shift reduces the T3:rT3 ratio and serves as a key biomarker of illness severity in both acute sepsis and chronic inflammatory conditions.

Mechanistic explanations

• Transcriptional Repression: Cytokines like TNF-α and IL-1β activate nuclear factor-kappa B (NF-κB) and activator protein 1 (AP-1) pathways. These transcription factors directly repress the transcription of the DIO1 and DIO2 genes, suppressing deiodinase activity.
• Oxidative Stress and Cofactor Depletion: IL-6 triggers intracellular oxidative stress and depletes glutathione (GSH). Because D1 and D2 enzymes rely on GSH-dependent cofactors for catalysis, this depletion functionally blunts the conversion of T4 to active T3.
• Shunting to rT3: Inflammation simultaneously suppresses D1-mediated rT3 clearance and upregulates type 3 deiodinase (D3), which actively converts T4 into rT3.
• Impaired Receptor Signaling: IL-1β downregulates the transcription of thyroid hormone receptor isoforms (TRα1, TRα2, and TRβ1) via NF-κB-dependent pathways. Furthermore, lipopolysaccharide (LPS) exposure during severe inflammation can physically compete with T3 for receptor binding, further reducing TR signaling.

Bottom line

• Systemic inflammation induces a state of tissue-level hypothyroidism by transcriptionally and functionally silencing deiodinase activity, shifting pathway metabolism toward inactive rT3, and downregulating thyroid hormone receptors.