Introduction: Some patients on levothyroxine replacement display significant impairment in psychological well-being, compared with sex- and age-matched controls. Levothyroxine-treated patients can be assumed to derive T3 exclusively from deiodination of T4, which, in the central nervous system, is regulated by type II deiodinase (DII).
Objective: We investigated whether two recently identified polymorphisms in the DII gene (DII-ORFa-Gly3Asp and DII-Thr92Ala) are determinants of well-being and neurocognitive functioning and associated with a preference for replacement with a combination of T3 and T4.
Methods: Genotypes for both polymorphisms were determined in 141 patients with primary autoimmune hypothyroidism, adequately treated with levothyroxine monotherapy and participating in a randomized clinical trial comparing T4 therapy with T4/T3 combination therapy. Questionnaires on well-being and neurocognitive tests were performed at baseline.
Results: Allele frequencies in patients with primary hypothyroidism were similar to those of healthy blood bank donors (32.0 vs. 33.9% for DII-ORFa-Gly3Asp and 40.4 vs. 38.8% for DII-Thr92Ala). DII polymorphisms were not associated with measures of well-being, neurocognitive functioning, or preference for combined T4/T3 therapy.
Conclusion: The DII-ORFa-Gly3Asp and DII-Thr92Ala polymorphisms do not explain differences in well-being, neurocognitive functioning, or appreciation of T4/T3 combination therapy in patients treated for hypothyroidism.