Can inflammatory cytokines suppress deiodinase activity and lower T3 while TSH and free T4 stay normal?

Systemic inflammation triggers a profound alteration in thyroid hormone metabolism known as non-thyroidal illness syndrome (NTIS). This condition is characterized by a selective decline in peripheral triiodothyronine (T3) levels while central regulatory markers remain temporarily unaltered.

Mechanistic pathways of deiodinase suppression

Pro-inflammatory cytokines, including interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-α), and interleukin-1 beta (IL-1β), directly impair peripheral type 1 (D1) and type 2 (D2) iodothyronine deiodinases:

• Oxidative stress and thiol depletion: IL-6 induces cellular oxidative stress and depletes intracellular thiols, such as glutathione. This directly compromises the catalytic function of D1 and D2, which are thiol-dependent selenoenzymes.
• Transcriptional repression: TNF-α and IL-1β trigger the activation of downstream NF-κB and AP-1 pathways. NF-κB pathway activation downregulates hepatic DIO1 transcription and expression, while also blocking thyroid hormone receptor (TRβ1) signaling.

Peripheral and central thyroid hormone dynamics

The suppression of peripheral deiodinases leads to distinct systemic and central thyroid hormone patterns:

• Peripheral T3 reduction: Suppressed peripheral D1 and D2 activity directly decreases the conversion of thyroxine (T4) to active T3. Concurrently, inflammatory signaling upregulates type 3 deiodinase (D3), accelerating the clearance and inactivation of T3 and T4 into reverse T3 (rT3).
• Preserved TSH and free T4: Despite a marked drop in systemic T3, serum TSH and free T4 levels remain temporarily normal. This preservation occurs because cytokines upregulate D2 expression in hypothalamic and pituitary tanycytes via NF-κB-responsive elements in the Dio2 promoter. This central upregulation maintains local intracellular T3 concentrations, preventing a compensatory rise in TSH.

Bottom line

• Inflammatory cytokines suppress peripheral D1 and D2 deiodinases through NF-κB-mediated transcriptional repression and intracellular thiol depletion, driving a systemic low-T3 state, while selectively upregulating central pituitary D2 to maintain local T3 feedback and temporarily preserve normal TSH and free T4 levels.