Is the DIO1 rs2235544 TT genotype linked to less efficient T4-to-T3 conversion?

Thyroid hormone conversion is influenced by DIO1, which encodes type 1 deiodinase, a key peripheral enzyme involved in converting T4 to the more active T3. For rs2235544, population-level hormone patterns support an association with variation in this conversion proxy, but not a definitive TT-genotype effect.

Clinical and biochemical evidence

• In the principal association study, the C allele was linked to a higher free-T3/free-T4 ratio, higher free T3, and lower free T4. The free-T3/free-T4 association was highly significant (P = 3.6 × 10⁻¹³), although the effect size was modest—about 0.2 SD per C allele.
• A subsequent report, describing the locus as −34C>A, found that the A allele was associated with lower free T3, higher free T4, and a lower T3:T4 ratio. This biochemical profile is consistent with relatively less efficient peripheral T4-to-T3 conversion.

Mechanistic interpretation

• The hormone-ratio findings are biologically coherent with altered deiodinase-mediated conversion: less conversion would be expected to produce relatively higher T4 and lower T3.
• However, rs2235544 is intronic and may tag another functional variant rather than directly alter DIO1 expression or enzyme activity. Available studies used circulating hormone measures, not direct assays of DIO1 protein abundance or enzymatic function.

Clinical implications

• The TT-specific statement remains conditional because published results use C/A allele notation, and it has not been established here whether the reported T allele corresponds to the lower-conversion A allele under the relevant strand/assay convention.
• TSH was not associated with this locus in the principal meta-analysis. This genotype alone should not be used to infer thyroid disease, predict individual thyroid status, or change thyroid-hormone treatment.

Bottom line

• DIO1 rs2235544 plausibly relates to less efficient T4-to-T3 conversion when the genotype maps to the lower T3:T4-ratio allele, but a direct, allele-orientation-confirmed TT effect has not been established.