Can inflammation and oxidative stress lower active T3 availability?

Peripheral thyroid hormone metabolism is highly sensitive to systemic immune and redox states. Under physiological stress, the conversion of thyroxine (T4) to active triiodothyronine (T3) is suppressed through distinct molecular pathways.

Inflammatory suppression of deiodinases

• Pro-inflammatory cytokines, including interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-α), and interleukin-1 beta (IL-1β), directly downregulate the expression and functional activity of type 1 (D1) and type 2 (D2) deiodinases.
• IL-6 post-translationally suppresses D1 and D2 while simultaneously upregulating type 3 deiodinase (D3). This reciprocal shift accelerates the inactivation of T4 and T3 into reverse T3 (rT3), lowering active hormone levels.

Oxidative stress and glutathione depletion

• Deiodinases are highly redox-sensitive selenoproteins that rely on reducing equivalents from glutathione (GSH) to regenerate their catalytic active sites.
• Oxidative stress consumes and depletes cellular GSH pools, while directly oxidizing the critical active-site selenocysteine residues of D1 and D2. Without adequate GSH to return these residues to a reduced state, the enzymatic cycle stalls, halting peripheral T4-to-T3 conversion.

Bottom line

• Systemic inflammation and oxidative stress synergistically impair peripheral thyroid hormone activation by downregulating deiodinase expression and depleting the glutathione needed for their catalytic regeneration, mimicking the clinical presentation of non-thyroidal illness syndrome.