BACKGROUND: Treatment of hypothyroidism with 3,5,3'-triiodothyronine (T(3)) is controversial. A recent meta-analysis concludes that no evidence is present in favour of using T(3). However, the analysis included a mixture of different patient groups and dose-regimens. OBJECTIVE: To compare the effect of combination therapy with thyroxine (T(4)) and T(3) versus T(4) monotherapy in patients with hypothyroidism on stable T(4) substitution. Study design Double-blind, randomised cross-over. Fifty micrograms of the usual T(4) dose was replaced with either 20 microg T(3) or 50 microg T(4) for 12 weeks, followed by cross-over for another 12 weeks. The T(4) dose was regulated if needed, intending unaltered serum TSH levels. Evaluation Tests for quality of life (QOL) and depression (SF-36, Beck Depression Inventory, and SCL-90-R) at baseline and after both treatment periods. Inclusion criteria Serum TSH between 0.1 and 5.0 mU/l on unaltered T(4) substitution for 6 months. RESULTS: A total of 59 patients (55 women); median age 46 years. When comparing scores of QOL and depression on T(4) monotherapy versus T(4)/T(3) combination therapy, significant differences were seen in 7 out of 11 scores, indicating a positive effect related to the combination therapy. Forty-nine percent preferred the combination and 15% monotherapy (P=0.002). Serum TSH remained unaltered between the groups as intended. CONCLUSION: In a study design, where morning TSH levels were unaltered between groups combination therapy, (treated with T(3) 20 microg once daily) was superior to monotherapy by evaluating several QOL, depression and anxiety rating scales as well as patients own preference.