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SHORT-TERM STUDIES OF THE EFFECT OF 

 RADIOIODINE THERAPY FOR THYROTOXICOSIS 

 ON THE THYROIDAL IODIDE CLEARANCE RATE 



I. D. 1 noMAs, T. H. Oddie and F. F. Rundle 



Unit of Clinical Investigation, Royal North Shore Hospital, 



Sydney, N.S. W. 



Iodine uptake studies of patients who have received radioiodine therapy for 

 thyrotoxicosis give resuks which frequently cannot clearly be correlated with 

 the patient's clinical status. It has been concluded by some that thyroidal 

 iodine uptake behaves capriciously after the administration of therapeutic 

 radioiodine, the corollary being that uptake studies are of little value in the 

 follow-up of these patients. The discrepancy between the clinical picture and 

 the uptake is not surprising, since the uptake rate is related to one aspect of 

 thyroid function, that is, thyroid hormone formation, while clinical assess- 

 ment depends on an estimation of metabolic processes, which in turn reflect 

 the level of circulating thyroid hormone. Some of this hormone is already 

 formed before therapy is given, while some may be released from the thyroid 

 by the breakdown of tissue following therapy. 



It has been shown that by using a precise method of measuring thyroidal 

 iodide clearance rate\ it is possible to predict with reasonable accuracy the 

 patient's response (in terms of this clearance rate), to a given dose of radio- 

 iodine^. It has further been shown that the achievement of a normal thyroidal 

 iodide clearance rate will eventually be followed by a normal state of the 

 patient, judged by other means-^ These studies have hitherto utilized 

 assessment of the iodide clearance rate at least two months after therapy, 

 this minimum interval being required because of the technical difficulties 

 involved in using small tracer doses against the high background of a 

 therapeutic dose. 



Using methods described elsewhere^ the present studies have followed the 

 variations in iodide clearance rate (iodine uptake), in sixteen thyrotoxic 

 patients during the first month after radioiodine therapy, estimations of 

 clearance rate being made 5 days, 13 days and 28 days after the giving 

 of the dose. 



The time at which the greatest lowering of uptake occurred and the level 

 of this uptake in relation to the usual diagnostic ranges have been noted. The 

 observations have been grouped according to whether a second therapeutic 

 dose proved necessary on follow-up. 



RESULTS 



Requiring further therapy (9 patients) 

 Time of lowest observed uptake 



5 days: 1 j^atient 

 13 days: 7 patients 

 28 days: 1 patient 

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