Thyroid dose reconstruction is most accurate when using the results of direct thyroid measurements, in which gamma radiation emitted by radionuclides present in the thyroid and in the remainder of the body is recorded by means of a radiation detector positioned against the neck. A large number of such measurements were performed in Belarus in May-June 1986. Owing to the short half-life of 131I and to the intake and accumulation of caesium radioisotopes (mainly 134Cs and 137Cs) in the body, the thyroid doses derived from thyroid measurements made after the beginning of June 1986 have so far been often considered to be unreliable. To evaluate the influence of the caesium radioisotopes to the signal recorded by an instrument performing measurement of 131I activity in the thyroid, a Monte Carlo method was used to calculate the calibration factors of that instrument. These calculations were made for males of six reference ages: newborn, 1, 5, 10 and 15 years old, and adult. The calibration factors were combined with estimated time-dependent intake functions for 131I and caesium radioisotopes. The fractions of the instrument indications that were due to 131I in thyroid were thus estimated as a function of the age of the subject that was measured and of the time elapsed since the accident. Using this information when processing the thyroid measurements made in May 1986 would improve the accuracy of the thyroid dose estimates, and may make it possible to use a larger proportion of the thyroid measurements made in June 1986.