The aim of this study was to evaluate parameters of thyroid function in patients with primary hypothyroidism receiving either monotherapy with L-T4 or combination L-T4+L-T3. Fifty-eight women with primary hypothyroidism receiving L-T4 were enrolled in the study. The patients were randomised into two groups: Group 1 (n=42) patients continued monotherapy with L-T4, and Group 2 (n=16) patients were switched to combined therapy with L-T4+L-T3 (25 microg L-T4 was replaced with 12.5 microg L-T3). The final examination was carried out 6 months thereafter. There was also a third group of 20 healthy women (control group). Under monotherapy with L-T4, serum FT4 levels were higher (p < 0.05) and FT3 lower (p < 0.05) than in the control group, while the monotherapy subgroup of patients with low-normal TSH had serum FT4 levels higher than in the control group (p < 0.05). Serum FT4 under combined therapy was significantly lower than in both control and monotherapy groups. FT3 levels did not differ between the two groups of combined and monotherapy subjects; the highest FT3 levels were in the control group. L-T4 replacement therapy is associated with non-physiologically high FT4 and low FT3 levels. Therapy with L-T3 once a day does not simulate the normal production of T3 by the thyroid.