The aim of this investigation was determination of salmon calcitonin (sCT) influence on changes of thyroid hormones levels in serum. The investigation was carried out on a group of 26 patients. The group was divided in two different ways: into people younger and older than 65 years, and into men and women. Investigation protocol consists of 4 days. On the 1st day patients received 100U of SC, on the 2nd day--100U of SC and 0.4 mg TRH, on the 3rd day--0.4 mg TRH, on the 4th day--0.9% solution of NaCl and 5% solution of human albumin. T3, T4 fT3 and fT4 concentrations in serum were determined by RIA method.