Nicotinamide (niacin) is very useful substance in treatment of many kinds of diseases. For the decades the main indications for application of niacin were lipid disorders. There are studies confirming that niacin increases thyroid radiosensitivity to 1-131. The radioiodine therapy is common method of treatment of hyperthyroidism (Graves' disease, toxic nodular goitre), large euthyroid goitre and differentiated thyroid cancer. In some studies with Wistar rats there is shown that niacin potentiates the effect of 1-131 by increasing thyroid blood flow, what results better effects of radioiodine treatment. Moreover niacin in therapeutic doses decreases serum thyroid hormone levels (mainly total thyroxine and thyroxine-binding globuline). In case of large goitre, when the calculated radioisotope dose exceeds ambulatory limits, the patient must be hospitalized. There are patients with low radioiodine uptake and radiosensitivity of thyroid, radiosensitizers can be utilized for this purpose. Recently obtained results of studies are showing that niacin can be used as radiosensitizer. It is making possible ambulatory treatment of patients with large goitres and low radioiodine sensitive thyroid. The radioiodine therapy with niacin could become shorter, less expensive and more safety for patients.