Does systemic inflammation reduce deiodinase activity and lower T4-to-T3 conversion?

Systemic inflammation significantly alters peripheral thyroid hormone metabolism, leading to a clinical presentation known as non-thyroidal illness syndrome (NTIS). This state is characterized by depleted circulating active triiodothyronine (T3) levels despite normal thyroxine (T4) levels.

Cellular and molecular mechanisms

• Deiodinase suppression: Pro-inflammatory cytokines, specifically interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-α), directly suppress the expression and catalytic activity of peripheral type 1 (D1) and type 2 (D2) deiodinases.
• NF-κB signaling: Systemic inflammation activates the NF-κB transcription factor pathway. Active NF-κB directly modulates deiodinase genes, suppressing DIO1 transcription and altering DIO2 expression.
• Inactivation pathways: Alongside the downregulation of D1 in the liver and kidneys and D2 in skeletal muscle, inflammatory states can upregulate the inactivating type 3 deiodinase (D3) enzyme, accelerating the clearance of active thyroid hormones.

Clinical manifestations and markers

• Surrogate markers: Elevated C-reactive protein (CRP) serves as a reliable clinical surrogate marker for the active cytokine-driven environment that suppresses peripheral thyroid conversion.
• Hormonal profiles: This coordinated enzymatic shift lowers the rate of T4-to-T3 conversion. The resulting drop in circulating T3 levels presents clinically as a low T3 pattern with normal or low-normal T4 and inappropriately uncompensated thyroid-stimulating hormone (TSH) levels.

Bottom line

• Systemic inflammation, signaled by elevated CRP and cytokines, activates NF-κB to suppress D1 and D2 deiodinase activity while promoting T3 inactivation. This impairs peripheral T4-to-T3 conversion, directly driving the low T3 pattern characteristic of non-thyroidal illness syndrome.