Can low-grade inflammation suppress androgen signaling and lower free T3?

Based on a review of the clinical and mechanistic evidence, the claim is fully supported. Low-grade inflammation directly impairs both the central regulation of the HPG (hypothalamic-pituitary-gonadal) axis and the peripheral activation of thyroid hormones, establishing a clear link between systemic inflammatory states, diminished testosterone, and low free T3 levels.

HPG Axis Suppression & Low Testosterone

• Central Suppression: Chronic low-grade systemic inflammation directly impairs the HPG axis. Pro-inflammatory cytokines, specifically tumor necrosis factor-alpha (TNF-$\alpha$) and interleukin-1 beta (IL-1$\beta$), act on the hypothalamus and pituitary gland to suppress the pulsatile secretion of Gonadotropin-Releasing Hormone (GnRH) and Luteinizing Hormone (LH).
• Peripheral Impairment: Beyond central nervous system effects, inflammatory cytokines also act directly at the level of the testes. TNF-$\alpha$ and IL-6 inhibit Leydig cell steroidogenesis, disrupting the enzymatic pathways required to synthesize testosterone.
• Bidirectional Modulation: Testosterone itself exerts protective, anti-inflammatory actions by suppressing the production of these same pro-inflammatory cytokines. Consequently, inflammatory signaling drives a feed-forward cycle that maintains low testosterone.

Impaired Thyroid Hormone Activation & Lower Free T3

• Deiodinase Dysregulation: The thyroid gland predominantly secretes the inactive prohormone thyroxine (T4). Biologically active triiodothyronine (T3) is generated peripherally via Type 1 (D1) and Type 2 (D2) deiodinase enzymes. Pro-inflammatory cytokines (IL-1$\beta$, TNF-$\alpha$, and IL-6) strongly downregulate the expression and activity of these activating deiodinases.
• Enhanced Inactivation: Simultaneously, inflammatory signaling often upregulates Type 3 deiodinase (D3), which actively degrades T4 and T3 into inactive metabolites, such as reverse T3 (rT3). This dual mechanism—impaired activation and accelerated clearance—directly results in lower circulating free T3 levels, a state often clinically referred to as Non-Thyroidal Illness Syndrome (NTIS) or euthyroid sick syndrome.

Bottom line

Systemic low-grade inflammation acts as a common upstream driver that simultaneously downregulates reproductive and metabolic axes. It suppresses hypothalamic-pituitary-gonadal signaling and Leydig cell function to lower testosterone, while concurrently inhibiting peripheral deiodinase activity to impair the conversion of T4 to active free T3.