Does low selenium strain thyroid hormone conversion and increase oxidative stress?

Physiological impact on thyroid hormone metabolism

• Impaired T4-to-T3 conversion: Iodothyronine deiodinases (DIO1, DIO2, and DIO3) are selenoproteins requiring selenocysteine at their catalytic sites. Low selenium levels compromise the synthesis and catalytic activity of these enzymes, particularly peripheral DIO1.
• Strained reverse T3 clearance: Reduced DIO1 activity impairs the outer-ring deiodination of T4 to active T3 and limits the degradation and clearance of reverse T3 (rT3). This restriction directly disrupts peripheral thyroid hormone balance, elevating the systemic T4:T3 ratio.

Thyroidal antioxidant defense and tissue damage

• Loss of antioxidant buffering: Thyroid hormone synthesis inherently generates hydrogen peroxide ($H_2O_2$). Under low selenium conditions, the synthesis and activity of selenium-dependent antioxidant enzymes—specifically glutathione peroxidases (GPx) and thioredoxin reductases (TrxR/TXNRD)—are significantly reduced.
• Oxidative cellular injury: The loss of these protective enzymes allows unneutralized reactive oxygen species (ROS) to accumulate. This elevated thyroid oxidative stress induces lipid peroxidation (marked by elevated malondialdehyde [MDA]), oxidative DNA damage (marked by 8-hydroxy-2'-deoxyguanosine [8-OHdG]), thyrocyte apoptosis, and secondary activation of autoimmune thyroiditis.

Bottom line

• Adequate selenium is mechanistically required to sustain the deiodinases and antioxidant enzymes that govern peripheral thyroid hormone activation and protect the thyroid gland from metabolic $H_2O_2$ accumulation. Ensuring sufficient selenium status is vital for supporting efficient T4-to-T3 conversion and safeguarding thyroid tissue from cumulative oxidative damage.