Does the DIO1 rs2235544 A allele lower circulating T3 by altering T4-to-T3 conversion?

The DIO1 gene encodes deiodinase type 1 (DIO1), a selenium-dependent enzyme that catalyzes the peripheral deiodination of thyroxine ($T_4$) into the active thyroid hormone, triiodothyronine ($T_3$). The DIO1 rs2235544 genetic polymorphism significantly influences baseline variation in peripheral thyroid hormone metabolism.

Clinical evidence

• Thyroid Hormone Ratios: Large-scale cohort studies and meta-analyses comprising over 3,000 individuals show that the rs2235544 polymorphism is strongly linked to peripheral thyroid status. The C allele is associated with increased enzyme activity. Consequently, individuals carrying the rs2235544 A allele exhibit relatively lower circulating free $T_3$ ($fT_3$) levels and a lower $fT_3/fT_4$ ratio.
• Statistical Significance: The genetic link between rs2235544 and the $fT_3/fT_4$ ratio is highly robust, reaching genome-wide significance in meta-analyses ($P = 3.6 \times 10^{-13}$).
• Clinical Presentation: The phenotypic impact of this variant is typically subtle. Most individuals carrying the A allele maintain a clinically euthyroid state with normal thyroid-stimulating hormone (TSH) levels, as the genetic variation explains baseline inter-individual setpoints rather than causing overt thyroid disease.

Mechanistic explanations

• Deiodinase Activity: The DIO1 enzyme is a major pathway for the outer-ring deiodination of $T_4$ to $T_3$. The rs2235544 C allele increases DIO1 enzymatic activity, driving more efficient conversion.
• Altered Conversion Rate: Conversely, the A allele is associated with relatively decreased DIO1 activity. This reduced enzymatic capacity alters the peripheral conversion process, resulting in less efficient clearance of $T_4$ and a diminished rate of $T_3$ production. This mirrors mild states of reduced peripheral deiodination, leaving carriers of the A allele with a lower baseline $T_3/T_4$ ratio.

Bottom line

• The DIO1 rs2235544 A allele is associated with less efficient peripheral conversion of $T_4$ to active $T_3$ due to reduced deiodinase type 1 activity, resulting in statistically lower circulating $T_3$ levels and a lower $T_3/T_4$ ratio compared to carriers of the C allele.