The content of medium-weight molecular peptides (MMP), lipid peroxidation (LPO) activity, the profile of blood thyroid hormones, and radioactive iodine absorption by the thyroid were determined in 104 patients with light, moderate and grave diffuse toxic goiter (DTG). The content of MMP in the blood correlated with the gravity of DTG and thus may serve an additional test for its determination. Depending on the treatment 66 patients with DTG of medium gravity were distributed into 5 groups: group I included patients given mercazolyl, group II patients on lithium carbonate, group III patients who received mercazolyl and thymalin, group IV patients on mercazolyl and piracetam, and group V included patients given mercazolyl, thymalin and piracetam. A study was made of the blood content of MMP and LPO activity before therapy and after euthyroidism attainment. The treatment with the use of mercazolyl, thymalin and piracetam was found to produce a beneficial effect on the parameters under study and the clinical course of DTG.