Thank you so much for providing this service. My question is about the safety of I131 therapy in a patient renal dysfunction. Her serum creatinine runs from 220umol/L to 310umol/L. (2.5-3.5mg/dL). She is a 65 year old woman who is hyperthyroid on the basis of an autonomously functioning nodule in a longstanding multinodular goitre. I recognize that with her renal dysfunction she may have a larger iodine pool. I also recognized that the I131 should take longer to be excreted. She is not on renal dialysis as she does not have endstage renal failure. Does the I131 therapy have the potential to worsen her already tenuous renal function? Thank you for your time!
Lois Donovan M.D. FRCP(c)
Univ. of Calgary
Your question raises several interesting issues which are probably often ignored. I think the possibility of significant direct radiation damage to the kidney is highly unlikely, since the kidney can probably withstand doses that exceed 1000 rads and survive. Another aspect is the potential radiation dose to the whole body. I presume that in the presence of a thyrotoxic nodule the RAIU, even in the face of renal failure, is proably +/- 20% or so. You are probably planning to give a dose in the order of 20-30mCi. (You can provide the details if you wish.) The body dose from such treatment comes both from the transient iodide phase and the more prolonged metabolism of labeled hormone. With normal iodide excretion half-time of 1 day it is small. However if excretion is very prolonged due to renal failure, the body dose could be significant. It could reach 100-200 rads depending on dosage, body size, and turnover rate. Most likely (assuming a 30 mCi dose and normal size and iodide half time of 3 days) it would be in the order of 100 rads and safe. Calculating the exact dose is complicated. To be absolutely safe, the ideal method would be to do a tracer uptake and whole body turnover curve counting the % dose retained each day. Probably your Nuclear Medicine people can help calculate the body dose. I will also ask Dr Kenneth Ain to repond to your query.
Leslie J De Groot,MD
My experience with I-131 administration, in the context of renal dysfunction, is mostly with therapy of metastatic thyroid carcinoma. In such patients, additional factors make dosing issues even more complicated. The induction of a sufficiently hypothyroid state to effect radioiodine uptake in tumor metastases decreases the GFR of a patient to around one third of what it would be in the euthyroid state. In these circumstances, using whole body and blood dosimetry analysis (setting the maximum red marrow exposure at 200 REM), in elderly women with chronic renal insufficiency (yet not so bad as to need dialysis), the lowest I-131 dosage I've seen to be associated with the red marrow radiation exposure limit is in the range of 95 mCi. In this euthyroid patient from Calgary, with chronic renal insufficiency, I-131 administration in the range if 20 - 30 mCi seem unlikely to draw near to any level of significant marrow toxicity. As stated by Dr. De Groot, the kidney itself is not likely to be damaged by this radiation. Also, with the considerable re-uptake of I-131 into autonomous regions of her thyroid from her reduced renal clearance, she might not require dosages as high as 20 mCi to effect a resolution of thyrotoxicosis (this could be modeled with a small I-131 tracer dose). Lastly, it should always be remembered that surgical thyroidectomy is a reasonable option in some patients, producing additional benefits of: rapid restoration of euthyroidism, elimination of potential thoracic outlet obstruction, and the opportunity to obtain histological analysis of the goiter.
Dr Kenneth B Ain