Can DIO1 and DIO2 polymorphisms affect thyroid hormone conversion and T3 availability?

Genetic variations in the deiodinase enzymes, DIO1 and 0IO2, play a crucial role in regulating thyroid hormone bioactivity, particularly in patients on thyroid hormone replacement therapy.

Clinical and physiological evidence

• The DIO1 rs2235544 C-allele is associated with increased type 1 deiodinase (D1) activity, which increases peripheral thyroxine (T4) to triiodothyronine (T3) conversion, resulting in a higher free T3 to free T4 (FT3/FT4) ratio and lower reverse T3 (rT3).
• Conversely, the DIO1 rs11206244 variant reduces D1 expression and lowers conversion capacity.
• The DIO2 rs225014 (Thr92Ala) polymorphism impairs type 2 deiodinase (D2) catalytic efficiency, leading to significantly lower circulating FT3 levels and a reduced T3/T4 ratio, especially in athyreotic patients or those on levothyroxine (L-T4) monotherapy.

Intracellular and neuropsychological mechanisms

• The DIO2 Thr92Ala variant impairs local, tissue-specific intracellular T4-to-T3 conversion within highly D2-dependent tissues such as the brain and skeletal muscle.
• This localized intracellular T3 deficit can trigger or exacerbate persistent neuropsychological symptoms—including fatigue, cognitive dysfunction, and altered mood—even when systemic thyroid-stimulating hormone (TSH) and FT4 levels remain within normal clinical reference ranges.

Bottom line

• Bottom line: Polymorphisms in DIO1 (rs2235544, rs11206244) and DIO2 (rs225014) directly alter deiodinase enzymatic activity, causing reduced peripheral T4-to-T3 conversion and localized intracellular T3 deficits that can manifest as persistent fatigue and cognitive symptoms during standard T4 monotherapy.