Physical and radiobiological bases of the use of 125I in the management of thyrotoxicosis.

Increasing interest is recently shown in the use of 125I as an alternative isotope to 131I for the management of thyrotoxicosis based on the postulate that there is a relative sparing of the reproductive integrity of the thyroid follicular cell and a consequent possibility of smaller incidence of hypothyroidism after therapy. A review of the dosimetric, radiobiological anc clinical aspects of the use of 125I are presented here. For the same activity though 125I gives smaller mean gland dose than 131I, the dose computations at microscopic level have revealed that there is a preferential irradiation of the apical membrane compared to the nucleus of the follicular cell. The ratio of the dose to the apical membrane and that to the nucleus increases with the decrease of the percentage colloid mass of the gland. Radiobiological significance of this non-uniform dose distribution across a follicular cell, with 125I , is brought out using rat thyroid as the biological system. For the same mean gland dose the follicular cell survival is larger with 125I than with 131I. 24 H radioiodine uptake is reduced in case of 131I treatment whereas it is not affected with 125I. Pilot clinical trials using 125I for the management of thyrotoxicosis are underway in some centres. Preliminary results from centres using doses 3--4 times that of the conventional 131I dose are not very different from those with conventional 131I therapy. Centres that used doses same as or less than the conventional 131I doses, reported smaller incidence of hypothyroidism.