Acute effects of iodine on the stimulated rat thyroid

Rats fed a low iodine diet (LID) for 5 weeks were given 125I as NaI to prelabel their thyroids; 24 h later, they were injected ip with 1 mg KI. They were then killed at different time intervals of up to 4 h. Thyroids of control animals not injected with KI were hyperplastic, the tissue showed microfollicles devoid of colloid, thyroglobulin was poorly iodinated, and thyroid peroxidase activity was increased. Within 4 h after KI injections, there was 1) a decrease in thyroid weight, 2) expansion of the follicular lumens of the hyperplastic in thyroid tissue and accumulation of colloid, 3) an increase in the thyroglobulin iodine concentration, and 4) a decrease in the thyroid peroxidase activity. Some of these changes started to occur as early as 60 min after KI administration. By that time, using the microprobe, most of the organically bound iodine was detectable inside the follicular cells. Radioiodine (125I) used to prelabel the thyroids was accumulated inside the follicles during the entire observation period. The serum TSH level was elevated and did not decrease after KI administration. It is concluded that excess iodine in LID rats may produce acute morphological and biochemical changes which seem to be a direct effect of iodine not mediated by modifications of TSH. The distribution of organically bound iodine, as determined by the microprobe, and the biochemical studies were interpreted as suggesting intracellular thyroglobulin iodination in these stimulated rat thyroids.