Can low zinc or selenium impair T3 signaling and contribute to fatigue?

Mechanistic pathways

• Selenium-dependent enzymes: Selenium is a core catalytic component of iodothyronine deiodinases (DIO1, DIO2, and DIO3) as selenocysteine, which directly regulates the peripheral conversion of thyroxine ($T_4$) to active triiodothyronine ($T_3$). Additionally, selenium-dependent glutathione peroxidases (GPx) and thioredoxin reductases neutralize the reactive hydrogen peroxide ($H_2O_2$) produced during thyroid hormone synthesis, shielding thyroid follicular cells from oxidative stress.
• Zinc-mediated transcription: Zinc functions upstream of hormone conversion. It serves as a vital structural cofactor for zinc-finger transcription factors, such as GATA4 and Kruppel-like factor 9 (KLF9), which bind and upregulate the DIO1 gene promoter. Zinc is also required for the structural integrity of nuclear thyroid hormone receptors that bind $T_3$, and it supports antioxidant defenses as a key component of copper/zinc superoxide dismutase (SOD1).

Clinical and physiological evidence

• Impact of deficiency on T3: Severe selenium deficiency impairs deiodinase function, resulting in elevated $T_4$-to-$T_3$ ratios. Clinical trials in elderly, selenium-deficient cohorts demonstrate that selenium repletion successfully restores peripheral thyroid hormone conversion and raises free $T_3$ levels. By contrast, zinc supplementation alone does not consistently alter free $T_3$ in individuals with adequate baseline zinc status.
• Thyroid signaling and fatigue: Reduced systemic $T_3$ availability compromises mitochondrial function and cellular respiration, leading to clinical fatigue and metabolic deceleration. Correcting an established zinc or selenium deficiency helps restore active $T_3$ signaling, though empirical improvements in subjective fatigue and thyroid parameters are rarely observed in individuals who are already trace-mineral replete.

Bottom line

• Selenium is directly required for catalytic thyroid hormone conversion and antioxidant defense, while zinc is essential for regulating deiodinase transcription and thyroid receptor binding. Insufficient levels of either element can impair active $T_3$ signaling and contribute to fatigue, though therapeutic benefits of supplementation are restricted to individuals with verified baseline deficiencies.