Can low-grade inflammation suppress T4-to-T3 conversion?

Systemic inflammatory states can profoundly alter thyroid hormone metabolism independently of thyroid gland function, leading to localized tissue-level thyroid hormone deprivation.

Mechanistic explanations

• Inactivation of activating enzymes: Pro-inflammatory cytokines, particularly interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-α), directly impair the Type 1 (D1) and Type 2 (D2) deiodinase enzymes responsible for converting T4 to active T3. IL-6-induced oxidative stress and intracellular glutathione depletion functionally inactivate the selenoprotein catalytic cycles of these enzymes.
• Transcriptional suppression: TNF-α recruits nuclear factor kappa B (NF-κB) signaling to repress hepatic DIO1 transcription, while IL-6 competes for essential transcriptional coactivators such as steroid receptor coactivator-1 (SRC-1), reducing D1 expression.
• Accelerated hormone degradation: Inflammatory signaling simultaneously upregulates Type 3 deiodinase (D3) expression. D3 actively degrades thyroid hormones by converting T4 into inactive reverse T3 (rT3) and converting active T3 into inactive T2, compounding the depletion of active thyroid hormone.

Clinical implications

• Euthyroid sick syndrome: This combined suppression of activating deiodinases and upregulation of inactivating pathways drives a state of relative tissue hypothyroidism, frequently referred to as Non-Thyroidal Illness Syndrome (NTIS).
• Deceptive serum markers: Because this pathology occurs in peripheral tissues, standard thyroid panels can be misleading. Patients typically exhibit a depressed free T3 to free T4 (fT3:fT4) ratio, while pituitary-derived thyroid-stimulating hormone (TSH) and circulating T4 levels remain within normal reference ranges.

Bottom line

• Low-grade inflammatory signaling directly impairs peripheral thyroid activation by downregulating D1/D2 and upregulating D3, leading to diminished active T3 availability at the cellular level despite normal circulating T4 and TSH.