Decreased quality of life is described more often in hypothyroid patients, who are treated with a synthetic form of thyroxine (L-T4), than in euthyroid controls. A combination of L-T4 and a synthetic form of triiodothyronine (L-T3) has been suggested; however, previous meta-analyses on unselected patients did not find any effect. Recent studies demonstrate, that the overall hypothalamic-pituitary-thyroid-tissue homeostasis could be more complex than previously suggested. Polymorphisms in deiodinase and thyroid hormone transporter genes could theoretically explain, why a minor subgroup of hypothyroid patients seem to have an effect of L-T4/L-T3 combination therapy.