Thyrotropin variations may explain some positive radioiodine therapy scans in patients with negative diagnostic scans.
Thyroglobulin (Tg) is a specific marker of residual thyroid cancer or tumor recurrence. In patients with elevated Tg levels and negative diagnostic radioiodine (131I) whole-body scans (dWBS), administration of a therapy dose may reveal foci that were not initially apparent. The aim of this study was to identify factors, other than 131I activity, which might explain why a post-therapy 131I whole-body scan is sometimes positive despite a negative dWBS.


METHODS
We reviewed data on all patients with elevated Tg levels and negative dWBS with 185 MBq 131I off-T4 at followup, who subsequently received an empiric therapy dose of 3700 MBq of 131I. During a 5-yr period, 22 patients met these criteria. 131I therapy could be given immediately after negative dWBS in 9 patients, with an average of 8 extra days of hypothyroidism. In the other 13 patients, therapy was given an average of 8 months later.


RESULTS
The therapy scan was negative in 16 patients, while it showed uptake in the thyroid bed in 5 patients and distant metastases in two. In the latter two patients, the TSH level was suboptimal at the time of dWBS (9 and 25 microIU/ml), and had risen to 34 and 70 microIU/ml respectively at the time of therapy. Overall, a positive scan following therapy occurred in 7 patients (6/9 patients treated immediately and 1/13 patients treated in a separate setting; p<0.01). In patients with positive therapy scans, the mean TSH level was 73 microIU/ml at the time of dWBS and 103.5 microIU/ml at the time of therapy (41% increase; p<0.05). In patients with negative therapy scans the mean TSH level was 84 microIU/ml at dWBS and 86 microIU/ml at the time of the therapy scan (2% increase).


CONCLUSIONS
Our study suggests that interval increase in TSH level with a longer period of stimulation may have contributed to making the whole-body scan positive at the time of therapy. Nowadays, patients with elevated Tg are directly given a therapy dose of 131I. Special care should be taken when preparing patients who have been on suppressive levothyroxine therapy for a long time, in order to avoid misclassifying the tumor as non-functioning.
Zanotti-Fregonara
P
P
DSV/I(2)BM/SHFJ/LMNRB, Commissariat à l'Energie Atomique, Orsay, France.
Keller
I
I
Rubello
D
D
Calzada-Nocaudie
M
M
Devaux
J Y
JY
Hindié
E
E
D000328:Adult; D018265:Carcinoma, Papillary, Follicular / Q000097:blood / Q000000981:diagnostic imaging*; D005188:False Negative Reactions; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D007457:Iodine Radioisotopes / Q000627:therapeutic use*; D008297:Male; D008875:Middle Aged; D011859:Radiography; D011877:Radionuclide Imaging; D012189:Retrospective Studies; D013964:Thyroid Neoplasms / Q000097:blood / Q000000981:diagnostic imaging*; D013972:Thyrotropin / Q000097:blood*; D051598:Whole Body Imaging
D007457:Iodine Radioisotopes; D013972:Thyrotropin