Can cortisol excess, inflammation, and selenium-zinc insufficiency lower free T3 while TSH and free T4 stay normal?

Peripheral thyroid hormone metabolism is highly sensitive to physiological stressors, where cortisol excess, systemic inflammation, and trace mineral deficiencies converge to impair deiodinase-mediated hormone conversion.

Mechanistic pathways of deiodinase alteration

• Hormonal and Inflammatory Suppression: Elevated cortisol and pro-inflammatory cytokines (including IL-6, TNF-α, and IL-1β) directly suppress the transcription and activity of type 1 (DIO1) and type 2 (DIO2) deiodinases in peripheral tissues. Simultaneously, they upregulate type 3 deiodinase (DIO3), accelerating the inactivation of T4 and T3.
• Nutritional Cofactor Insufficiency: Because deiodinases are selenoproteins, selenium insufficiency directly compromises the catalytic active sites of DIO1 and DIO2. Zinc further acts as a critical structural cofactor supporting thyroid receptor conformation and deiodinase function, compounding the peripheral conversion deficit when deficient.

Preservation of normal TSH and free T4

• Central Feedback Suppression: Despite low peripheral free T3, TSH remains within normal limits. Cortisol and inflammatory cytokines directly suppress hypothalamic TRH and pituitary TSH secretion, blunting the expected compensatory feedback response.
• Local Pituitary Compensation: Central DIO2 expression in the pituitary is preserved, maintaining local intracellular T3 levels and preventing the compensatory TSH surge typical of primary thyroid failure.

Bottom line

• Bottom line: Cortisol excess, systemic inflammation, and selenium-zinc insufficiency collectively impair peripheral T4-to-T3 conversion by downregulating DIO1/DIO2 and upregulating DIO3. Normal TSH and free T4 are maintained through central HPT axis suppression and localized pituitary deiodination, establishing a low free T3 pattern without typical markers of primary hypothyroidism.