In thyroid cancer patients, pre-treatment with rhTSH reduces retention of 131I
Reston, VA: Thousands of thyroid cancer patients are treated with Iodine 131 (131I ) each year. This life-saving therapy has effectively diminished and eliminated tumors, but there are lingering questions about the impact of 131I on non-diseased parts of the body. German researchers who recently conducted a large study to determine if the use of recombinant human thyroid-stimulating hormone (rhTSH) prior to 131I therapy affects radiation retention found that there is indeed a correlation, where the use of rhTSH significantly reduces 131I's effective half-life and therefore potential harmful effects.
The U.S. Food and Drug Administration approved rhTSH for diagnostic purposes in 1998 because of its ability to stimulate uptake of radioiodine into thyroid remnants and metastases of thyroid cancer, therefore improving diagnostic results. Doctors have also used rhTSH prior to 131I treatment as an alternative to withdrawing patients' thyroid hormone medication, because withdrawing this medication leads to a hypothyroid state, which can make patients very uncomfortable and affect their concentration. Recognizing both of these positive uses of rhTSH, researchers wondered how the use of rhTSH compared with withdrawal of hormone medication in impacting the retention of therapeutic doses of 131I.
The results of the study, which were published in the July 2003 issue of The Journal of Nuclear Medicine, found that the mean effective half-life of radioiodine was 26 percent shorter in patients who received rhTSH before therapy. The study examined 227 patients, 163 of whom were prepared for the therapy by thyroid hormone withdrawal and 64 of whom received rhTSH stimulation prior to therapy.
Patients subjected to thyroid hormone withdrawal retained 131I for a longer period of time because iodine is principally cleared through the kidney, and renal blood flow and filtration are reduced in the hypothyroid state. Prolonged exposure to radiation allows it to infiltrate blood and tissues, with possible serious side effects. Patients who received rhTSH maintained normal renal clearance rates. Because they cleared the 131I more rapidly, they had lower radiation levels in their blood and tissues.
As Richard J. Robbins, MD and Keith S. Pentlow, MD of the Memorial Sloan-Kettering Cancer Center in New York, NY point out in their invited commentary, the results of the study clearly show that rhTSH reduces whole body exposure to 131I. An important question, however, must still be answered: does rhTSH reduce the effectiveness of 131I in destroying the cancer being targeted?
"rhTSH Stimulation Before Radioiodine Therapy in Thyroid Cancer Reduces the Effective Half Life of 131I" was written by Christian Menzel, MD, PhD, Wolfgang T. Kranert, ScD, Natascha Döbert, MD, Michaela Diehl, MD, Thomas Fietz, ScD, Nadja Hamscho, MD, Uwe Berner, MD, and Frank Grünwald, MD, PhD, all from the Department of Nuclear Medicine at Johann W. Goethe University Hospital in Frankfurt, Germany.
Copies of the article are available to media upon request to Kimberly A. Bennett. Copies of the current and past issues of The Journal of Nuclear Medicine are available online at jnm.snmjournals.org. A yearly subscription to the journal is $170. A journal subscription is a member benefit of the Society of Nuclear Medicine. The Society of Nuclear Medicine is an international scientific and professional organization of more than 14,000 members dedicated to promoting the science, technology, and practical applications of nuclear medicine. The SNM is based in Reston, VA.