Can HPA-axis dysregulation impair T4-to-T3 conversion and lower free T3 even with normal reverse T3?

Mechanistic pathways of thyroid-HPA crosstalk

• Deiodinase dysregulation: Hypothalamic-pituitary-adrenal (HPA) axis activation and elevated cortisol directly suppress the activities of type 1 (D1) and type 2 (D2) deiodinases, the key enzymes responsible for converting prohormone thyroxine (T4) into active triiodothyronine (T3).
• Enzymatic shunting: Concurrently, stress and elevated cortisol levels upregulate type 3 deiodinase (D3), which inactivates thyroid hormone by converting T4 into reverse T3 (rT3) and degrading active T3.
• Role of DHEA-S: While DHEA-S serves as a major HPA-axis counter-regulator, direct evidence showing that it modulates human deiodinase kinetics is limited; its influence is primarily understood through its systemic antagonism of cortisol’s suppressive effects.

Deiodination patterns and reverse T3 levels

• Global monodeiodination inhibition: Although HPA-axis activation is classically associated with low free T3 and high rT3, an elevated rT3 is not a mandatory feature of this physiological response.
• Normal rT3 in low-T3 states: Under certain stress states, HPA-axis dysregulation can cause a global inhibition of T4 monodeiodination—reducing conversion to both active T3 and inactive rT3—rather than a selective metabolic shunt. Consequently, free T3 levels fall while rT3 levels remain entirely normal, meaning a normal rT3 level does not rule out stress-induced peripheral thyroid impairment.

Bottom line

• HPA-axis dysregulation and elevated cortisol impair peripheral T4-to-T3 conversion by suppressing D1 and D2 deiodinases and upregulating D3. Because stress can globally inhibit deiodination rather than preferentially shunting T4 to rT3, this low free T3 state and functional tissue hypothyroidism can readily occur even when reverse T3 remains within the normal range.