rs225015 — DIO2 DIO2 rs225015

DIO2 rs225015 — A Regulatory Switch in the Thyroid Hormone Conversion Gene

The DIO2 gene encodes type II iodothyronine deiodinase¹¹ type II iodothyronine deiodinase
A selenoprotein enzyme expressed in the brain, pituitary, thyroid, skeletal muscle, and heart that converts the prohormone T4 into the biologically active T3 by removing one iodine atom from the outer ring, the enzyme responsible for locally activating thyroid hormone in the brain, heart, muscle, and pituitary gland. The thyroid itself secretes mostly T4 (thyroxine), which is biologically inert until converted to T3 (triiodothyronine) by deiodinase enzymes in peripheral tissues. DIO2 is the dominant enzyme for this conversion in the central nervous system, where it supplies up to 80% of intracellular T3.

The rs225015 variant sits in the 3' untranslated region (3' UTR)²² 3' untranslated region (3' UTR)
A non-coding stretch of mRNA downstream of the stop codon that regulates mRNA stability, translation efficiency, and response to regulatory proteins like microRNAs of the DIO2 gene on chromosome 14. Unlike the nearby Thr92Ala variant (rs225014), which alters the enzyme protein itself, rs225015 acts at the regulatory level — potentially changing how much DIO2 protein is produced from the gene. This 3' UTR position may affect mRNA stability or microRNA binding³³ mRNA stability or microRNA binding
Changes in the 3' UTR can alter which regulatory microRNAs bind to the transcript, changing its half-life and the efficiency with which ribosomes translate it into protein and thereby modulate tissue-level DIO2 enzyme abundance.

Because DIO2 is also a selenoprotein⁴⁴ selenoprotein
It contains selenocysteine at its catalytic center, an amino acid that requires selenium for incorporation and is essential for the enzyme's deiodination activity, selenium status forms a second layer of influence on DIO2 function beyond genetics — making dietary selenium particularly relevant for anyone with variants that already challenge optimal thyroid hormone conversion.

The Mechanism

The DIO2 gene is located on the minus (reverse) strand of chromosome 14. The rs225015 variant is a G-to-A substitution at position 80,201,236 on the GRCh38 plus strand, corresponding to a C-to-T change in the coding-strand notation used in many publications. The variant affects the 3' UTR of all major DIO2 transcript isoforms NM_000793, NM_013989, and NM_001324462⁵⁵ NM_000793, NM_013989, and NM_001324462
Three splice variants of DIO2 encoding the same catalytic protein with slightly different untranslated regions without altering the protein sequence.

The biological impact of the A allele is not fully characterized at the molecular level — this is a regulatory variant rather than a functional coding change, and its mechanism operates through changes in gene expression rather than enzyme structure. What clinical studies show is that rs225015 genotype is associated with differences in levothyroxine dose requirements and TSH levels⁶⁶ levothyroxine dose requirements and TSH levels
Arici et al. Endocrine Journal 2018 in hypothyroid patients, implying that the variant affects the amount of functional DIO2 enzyme available in tissues.

The Evidence

A 2018 study by Arici et al. in a Turkish hypothyroid cohort⁷⁷ Arici et al. in a Turkish hypothyroid cohort
Endocrine Journal 2018 found that rs225015 genotype was associated with TSH levels and differences in optimal levothyroxine dose, with GG genotype patients requiring lower doses — suggesting these individuals may have higher DIO2 expression and thus more efficient T4-to-T3 conversion, making them more sensitive to exogenous T4. The A allele, by this model, would reduce DIO2 expression and blunt peripheral conversion.

A large UK Biobank analysis of 18,761 levothyroxine-treated patients and 360,534 controls⁸⁸ UK Biobank analysis of 18,761 levothyroxine-treated patients and 360,534 controls
Jensen et al. J Clin Endocrinol Metab 2024 found that the minor A allele showed a nominally significant association with financial dissatisfaction in LT4-treated individuals — a proxy for reduced quality of life that may reflect persistent hypothyroid-like symptoms. However, this study found no significant association with psychological well-being, cognitive function, or cardiovascular risk factors overall. The authors concluded that rs225015, in isolation, does not robustly explain persistent symptoms in levothyroxine-treated patients, echoing findings for the more-studied rs225014 variant in the same gene.

A Danish randomized crossover trial of 45 hypothyroid patients⁹⁹ Danish randomized crossover trial of 45 hypothyroid patients
Carlé et al. Eur Thyroid J 2017 examined combined DIO2 and MCT10 polymorphism burden and found that treatment preference for T3+T4 combination therapy increased with genetic burden — though the study was insufficiently powered to identify effects of rs225015 independently from rs225014 and rs17606253 (MCT10).

Observational data from cardiovascular cohorts suggest additional associations: a study of 290 acute myocardial infarction patients¹⁰¹⁰ 290 acute myocardial infarction patients
Brozaitiene et al. Genet Test Mol Biomarkers 2018 found an association between rs225015 genotype and diabetes mellitus comorbidity, and 168 ischemic stroke patients¹¹¹¹ 168 ischemic stroke patients
Taroza et al. J Stroke Cerebrovasc Dis 2020 were genotyped as part of thyroid axis variant analysis. These associations are exploratory and have not been independently replicated at genome-wide significance.

Practical Actions

For individuals with the AA genotype who are on levothyroxine therapy and experience persistent hypothyroid-like symptoms (fatigue, brain fog, cold intolerance) despite normal TSH, rs225015 could be one contributing factor — particularly when combined with the nearby rs225014 (Thr92Ala) variant. The evidence for rs225015 alone is more limited than for Thr92Ala, and a thorough conversation with an endocrinologist about free T3 levels (not just TSH) is the appropriate next step before considering treatment changes.

Because DIO2 is a selenoprotein, maintaining adequate selenium status is specifically relevant for DIO2 function. The enzyme requires selenocysteine at its catalytic center, and while DIO2 receives prioritized selenium supply during deficiency¹²¹² DIO2 receives prioritized selenium supply during deficiency
Köhrle & Frädrich Free Radic Biol Med 2022, severe selenium deficiency does impair activity. Monitoring serum selenium and considering supplementation with selenomethionine (the organic form with better bioavailability) is a practical step for AA carriers, especially in regions with selenium-poor soils.

Interactions

rs225015 sits in the same gene as rs225014 (DIO2 Thr92Ala), and the two variants are in partial linkage disequilibrium. Studies examining both together suggest additive effects on levothyroxine response: GG/TT haplotypes (both wild-type) show the most efficient T4-to-T3 conversion, while carrying risk alleles at both positions compounds the challenge. However, no published study has characterized the rs225015 + rs225014 haplotype effect with sufficient statistical power to quantify the combined impact precisely.

The MCT10 transporter variant rs17606253 represents a functionally complementary interaction: MCT10 shuttles thyroid hormones into cells, and DIO2 then activates T4 to T3 inside the cell. Variants impairing both transport and conversion create a compound barrier to adequate intracellular T3 that neither variant alone fully predicts. The Carlé 2017 RCT showed 100% treatment preference for T3+T4 in individuals carrying polymorphisms in both genes.

For individuals with AA at rs225015 and CC at rs225014 (the Thr92Ala variant), both the regulatory and catalytic aspects of DIO2 are potentially compromised — this combination may warrant the strongest consideration for combination T3+T4 therapy evaluation.