The authors studied the serum level of T3, T4 and TTH in 30 euthyroid patients with chronic renal insufficiency (CRI), distributed in three groups of 10 patients. I group includes patients with CRI II and III stage without dialysis treatment, patients with CRI are included in the II group, being under hemodialysis treatment from 5 to 12 months, and in III group--patients dialized three and more years. Low average values of T3--0.65 mg/ml were established only in the first group; in 8 patients, out of 10 examined, the values were under the lower limit of the norm. Though the T4 values in the first group were within the limits of the norm (5.87 mkg/100 ml), they were under the average normal values (8.5 mkg/100 ml) and lower, with a statistical significance (pt less than 0.025) as compared with those of the other two groups. The values of T3 and T4 in both groups dialyzed patients were within the limits of the norm regardless of the duration of dialysis. In none of the patients from the three groups examined, deviations in TTH level were found. The authors drew the conclusion that biochemical hypothyroidism, manifested with low T3 values, normal TTH level and a tendency of T4 decrease was observed in nondialyzed patients even with CRI II stage (creatine 6.8 mg%). Biochemical hypothyroidism abates with the adequate and effective hemodialysis treatment, suggesting that uremic toxins play and essential role in its development. It was stressed that abatement could be used as a criterion of adequate and effective dialysis programme and a reliable rehabilitation of the patients, under chronodialysis treatment.