The energy economy induced by a reducing diet and manifested by a reduced energy output at rest threatens greatly the long-term success of reducing regimes. The authors investigated the effect of a four-week therapy of obese patients by a low energy protein diet with an energy value of 1560 kJ on the resting metabolic rate (RMR), the serum concentration of thyroid hormones and TSH and on the Achilles tendon reflex time. Four weeks' treatment with a low energy protein diet caused a significant drop of the energy output at rest in the control group of obese men (by 17.5%), as well as in the control group of obese women (by 16.9 per cent). In the group of obese women who were given during the last two weeks of treatment depot methyl xanthine derivative, Euphilline (Byk Gulden), the energy output at rest did not decline as a result of dietetic treatment and the decline of the BMI was significantly higher than in the control group of obese women, although the two groups did not differ in the initial BMI, mean age and the degree of physical activity during treatment. With the exception of the control group of men where the serum triiodothyronine level declined, in the investigated obese subjects no significant changes of the investigated thyroid function parameters were recorded. The syndrome of low T3 thus does not participate in a substantial way in the drop of the metabolism at rest during treatment with a low energy protein diet, obviously due to the intake of 50 g carbohydrate per day.(ABSTRACT TRUNCATED AT 250 WORDS)