Does elevated cortisol suppress peripheral T4-to-T3 conversion?

Excessive cortisol levels, whether from physiological stress, medical conditions like Cushing’s syndrome, or pharmacological glucocorticoid use, exert a significant influence on thyroid hormone metabolism. The primary outcome of hypercortisolemia is a marked reduction in the conversion of the prohormone thyroxine (T4) into its biologically active form, triiodothyronine (T3).

Clinical and metabolic evidence

Elevated cortisol levels systematically shift thyroid metabolism away from active T3 production, a state often referred to as "low T3 syndrome" or non-thyroidal illness syndrome (NTIS).

• Reduced T3 Levels: In clinical studies of Cushing’s syndrome, between 24.6% and 68.4% of patients exhibit free T3 (FT3) levels below the reference range despite normal or near-normal T4 levels. This deficit typically resolves once cortisol levels are normalized through treatment.
• Exogenous Glucocorticoids: Administration of dexamethasone or hydrocortisone has been shown to rapidly decrease serum T3 concentrations in human subjects. A single dose of dexamethasone can significantly lower T3 within hours, demonstrating a direct effect on peripheral conversion independent of thyroid gland production.
• Metabolic Diversion: As T3 production falls, there is often a reciprocal increase in reverse T3 (rT3), an inactive metabolite. This creates a high rT3/T3 ratio, indicating that the body is prioritizing the inactivation of thyroid hormone over its activation.

Mechanistic explanations

The reduction in T3 availability is driven by changes in the activity of deiodinase enzymes, which are responsible for removing iodine atoms from thyroid hormones.

• Deiodinase Inhibition: Cortisol suppresses the peripheral activity of 5'-deiodinases, the enzymes required to convert T4 to T3. This inhibition occurs primarily in peripheral tissues like the liver and kidneys.
• Enzyme Specificity: While the overall metabolic effect is a decrease in T3, the transcriptional response of specific deiodinases is complex. Type 1 deiodinase (DIO1) activity in the liver and kidney is generally suppressed or inhibited by high cortisol. However, research into Type 2 deiodinase (DIO2) shows that glucocorticoids may actually increase its expression in specific tissues like the brain, pituitary, and brown adipose tissue, likely as a compensatory or tissue-specific regulatory mechanism.
• Type 3 Deiodinase (DIO3): The shift toward lower T3 availability is further exacerbated by the maintenance or induction of Type 3 deiodinase, which actively converts T4 into rT3 and degrades existing T3 into T2, effectively "shunting" thyroid hormone into inactive pathways.

Bottom line

Elevated cortisol significantly suppresses the peripheral conversion of T4 to T3 and increases the production of inactive reverse T3. While this is primarily mediated by the inhibition of Type 1 deiodinase activity in key metabolic organs, the effect on Type 2 deiodinase is more nuanced and tissue-dependent. Clinically, this manifests as reduced T3 availability and a metabolic state prioritized for energy conservation during stress.