Common Variation in the DIO2 Gene Predicts Baseline Psychological Well-Being and Response to Combination Thyroxine Plus Triiodothyronine Therapy in Hypothyroid Patients

Introduction: Animal studies suggest that up to 80% of intracellular T3 in the brain is derived from circulating T4 by local deiodination. We hypothesized that in patients on T4 common variants in the deiodinase genes might influence baseline psychological well-being and any improvement on combined T4/T3 without necessarily affecting serum thyroid hormone levels. Methods: We analyzed common variants in the three deiodinase genes vs. baseline psychological morbidity and response to T4/T3 in 552 subjects on T4 from the Weston Area T4 T3 Study (WATTS). Primary outcome was improvement in psychological well-being assessed by the General Health Questionnaire 12 (GHQ-12). Results: The rarer CC genotype of the rs225014 polymorphism in the deiodinase 2 gene (DIO2) was present in 16% of the study population and was associated with worse baseline GHQ scores in patients on T4 (CC vs. TT genotype: 14.1 vs. 12.8, P = 0.03). In addition, this genotype showed greater improvement on T4/T3 therapy compared with T4 only by 2.3 GHQ points at 3 months and 1.4 at 12 months (P = 0.03 for repeated measures ANOVA). This polymorphism had no impact on circulating thyroid hormone levels. Conclusions: Our results require replication but suggest that commonly inherited variation in the DIO2 gene is associated both with impaired baseline psychological well-being on T4 and enhanced response to combination T4/T3 therapy, but did not affect serum thyroid hormone levels. In subjects on thyroid hormone replacement, a D2 deiodinase genotype is associated with poorer psychological well-being and increased likelihood of improvement with addition of triiodothyronine.