Can low-grade inflammation reduce active T3 production?

Clinical and Physiological Evidence

• Deiodinase Suppression: Pro-inflammatory cytokines, specifically interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-α), and interleukin-1 beta (IL-1β), directly impair the peripheral conversion of thyroxine (T4) to active triiodothyronine (T3).
• Altered Thyroid State: This inhibition leads to a systemic shift toward a "low T3, high rT3" (reverse T3) profile, commonly seen in non-thyroidal illness syndrome. Clinical studies indicate that elevated levels of circulating IL-6 and TNF-α correlate strongly with decreased serum T3 concentrations and elevated rT3 levels, independent of primary thyroid gland pathology.

Mechanistic Pathways

• Transcriptional Repression: Pro-inflammatory cytokines suppress the transcription of the DIO1 and DIO2 genes, which encode the Type 1 and Type 2 deiodinase enzymes. This downregulates the synthesis of the enzymes responsible for generating active T3.
• Thiol Cofactor Depletion: Deiodination is a reductive process that requires sulfhydryl (thiol) donors, such as glutathione, to regenerate the active enzyme. Chronic inflammation induces oxidative stress, depleting intracellular glutathione pools and post-translationally limiting the catalytic capacity of D1 and D2.
• Upregulation of D3: In addition to suppressing activating enzymes, inflammatory signaling (specifically via IL-6) can upregulate Type 3 deiodinase (D3). This enzyme actively inactivates thyroid hormones, converting T4 to rT3 and T3 to T2, further steering the thyroid hormone economy away from active thyroid hormone action.

Bottom Line

• Systemic low-grade inflammation directly impairs T4-to-T3 conversion by transcriptionally repressing deiodinases 1 and 2, depleting necessary glutathione cofactors, and upregulating the inactivating deiodinase 3. This shifts the overall thyroid hormone economy toward the inactive metabolite rT3.