Adjuvant radioiodine therapy (RITh) for differentiated thyroid carcinoma is performed either with thyroid hormone withdrawal or with administration of recombinant human thyroid-stimulating hormone (rhTSH). Heterogeneous results have been obtained on the impact of the method of patient preparation on thyroid uptake and whole-body effective half-life. A higher radiation exposure using thyroid hormone withdrawal for several weeks compared to rhTSH was reported in prior studies. It was the aim to

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... amine if these findings are reproducible in a modern protocol with a short interval between surgery and RITh. Method: A retrospective study was performed on patients admitted for adjuvant radioiodine therapy for differentiated thyroid carcinoma at the University Hospital of Cologne over a five-year period from 2010. Dose rate measurements were analyzed for 366 patients and subgroup analyses were performed for papillary (PTC, n = 341) and follicular thyroid cancer (FTC, n = 25) patients, gender, length of hypothyroidism and normal versus decreased glomerular filtration rate (GFR). Results: Mean interval between surgery and RITh was 18 days for thyroid hormone withdrawal and 26 days for rhTSH (p < 0.01). The mean thyroid uptake was 4.2 ± 1.8 % for the 300 hypothyroid patients versus 3.8 ± 1.6% (p = 0.12) for the 66 rhTSH patients. Whole-body half-life in the hypothyroid group was significantly longer at 19.3 ± 7.7 h versus 16.4 ± 4.6 h in the rhTSH group (p < 0.01). Results were predominantly influenced by data from the largest subgroup, i.e. female PTC patients. Within this group, wholebody half-life was significantly shorter in the rhTSH treatment arm. Duration of hypothyroidism and a decrease in GFR < 60 ml/min/1.73 m 2 significantly influenced results with increased whole-body half-life occurring in the hypothyroid group. When patients returned for whole-body scintigraphy thyroid, half-life and whole-body half-life were significantly shorter in the rhTSH groups resulting in a low thyroid and remaining-body dose. Conclusions: With a shortening of the time between surgery and adjuvant radioiodine therapy, thyroid uptake is not significantly changed but wholebody half-life becomes longer in the hypothyroid group. Radiation exposure for most patients is not significantly different. However, patients with a hypothyroid phase of more than 4 weeks, and in particular those with a decreased GFR, experience higher radiation exposure.