[en] To determine the radiation dose delivered to the thyroid tissue from ¹³¹I in patients of well differentiated thyroid carcinoma, it is essential to quantitate accurately the parameters of radioactive iodine uptake (RAIU) and the biological half-life (BHL) OF¹³¹I in the tissue from the therapeutic dose. These parameters are rarely quantitated after therapy, and very often are assumed to be the same as that obtained by diagnostic dose, thus affecting the precise estimation off radiation dose. Hence, to have an improved estimate of radiation dose, we have standardised a technique using Gun monitor, to quantitate the RAIU and BHL of ¹³¹I in the thyroid tissue from the measured surface neck dose rates following therapy. The data of RAIU and BHL as determined using GUN monitor and the mass of the tissue as determined from the rectilinear scans, were used for calculation of the radiation dose in 30 treated patients, using MIRD methodology. Of the 30 patients, 22 showed total ablation an 8 showed partial ablation of the residual thyroid tissue. The radiation dose delivered to the tissue in the totally ablated (TA) group (27250 ± 22573 rads) was not significantly different (0.6>p>0.5) from those in the partially ablated (PA) group (21492 ± 29069 rads). Also, the effective half-life (EHL) of ¹³¹I in the tissue of TA group (62.20 ± 30.18 h) was not significantly different (0.4>p>0.3) from the PA group (75.20 ± 33.27 h). From this preliminary data, it appears that the treatment response to ¹³¹I in thyroid carcinoma patients is independent of the radiation dose and EHL of ¹³¹I in the thyroid tissue and it is the individual radiosensitivity of the tissue that determines the efficacy of treatment. (author). 1 ref