Can HPA-axis dysregulation lower active T3 availability by altering thyroid deiodinase activity?

Chronic activation of the hypothalamic-pituitary-adrenal (HPA) axis under stress initiates a coordinated endocrine response that directly alters peripheral thyroid hormone metabolism, shifting the body's metabolic setpoint.

Mechanistic pathways of deiodinase modulation

• Enzymatic redirection: Elevated circulating cortisol levels and stress-induced pro-inflammatory cytokines suppress the activity of type 1 (D1) and type 2 (D2) deiodinases, the enzymes responsible for converting thyroxine (T4) into active triiodothyronine (T3).
• Inactivation and rT3 accumulation: Concurrently, elevated cortisol upregulates type 3 deiodinase (D3). This endocrine shift accelerates T3 degradation and preferentially shunts T4 into inactive reverse T3 (rT3).
• Impaired clearance: Because D1 is suppressed, the clearance of rT3 is simultaneously reduced, resulting in elevated circulating rT3 alongside a pronounced deficit in systemic and tissue-specific T3.

Adaptive energy conservation

• Metabolic braking: T3 is the primary driver of cellular metabolism. The systemic reduction of active T3 limits mitochondrial oxygen consumption, decreases protein synthesis, and dampens anabolic signaling pathways.
• Resource reallocation: This downshift, characteristic of non-thyroidal illness syndrome (NTIS), serves as an adaptive "metabolic brake." During periods of severe stress, starvation, or critical illness, it allows the organism to conserve energy by redirecting resources away from thermogenesis and growth toward cellular defense and immune function.

Bottom line

• HPA-axis dysregulation and elevated cortisol suppress activating deiodinases (D1/D2) and upregulate inactivating D3, lowering active T3 availability and elevating rT3 to act as an adaptive, energy-conserving metabolic brake during chronic physiological stress.