Thyroid Uptake of Radioiodine Following Various Routes of Administration

Abstract

Thyroidal uptake of radioiodine following various routes of administration was studied in young sheep utilizing a double tracer technique and three detection systems. Iodine-125 and I131 were given simultaneously by various combinations of intravenous, subcutaneous, oral and topical routes. Thyroid burdens were determined using a whole body monitor with a 9 × 4 in. NaI crystal, a modified Geiger-Mueller tube assembly, a 2 × 2 in. NaI crystal and a 2 × 0.04 in. NaI crystal. Rates and per cent of uptake and effective half life were similar following the oral, intravenous and subcutaneous routes of administration. A lower uptake and a longer effective half life were seen following topical administration. Variations which may be associated with age and season were observed. No correlation could be made between the per cent thyroid uptake and the location or total area of skin involved with topical application. Estimated total radiation dose to the thyroid from oral administration was 6-8 rad/µc, while that from topical administration was 2-3 rad/µc.