Diagnostic 123I and 131I activities and radioiodine therapy. Effects on urinary iodine excretion in patients with differentiated thyroid carcinoma.
OBJECTIVE
Urinary iodine excretion (UIE) provides information about iodine supply and release. In the present study we investigated effects of the application of different radioiodine isotopes on UIE in patients with differentiated thyroid carcinoma (DTC).


METHODS
In 91 consecutive patients with DTC UIE, measured as iodine/creatinine ratio, was determined before and after application of 123I and 131I for diagnostic or therapeutic purposes. Additionally, remnant volume (V) was determined prior to therapy. Group A consisted of 33 patients with supposed successful ablation of DTC. These patients received 370 MBq 131I for diagnostic use and served as controls. 58 patients (group B) with remnants, relapses and metastases received 370 MBq 123I for diagnostics prior to therapy with 1.5-22.2 GBq 131I. Factors influencing individual changes in urinary iodine excretion (deltaUIE) were investigated by using non-parametric tests.


RESULTS
In group A UIE did not change significantly after application of 131I. As well, UIE remained unchanged after diagnostic application of 123I in group B. In contrast, UIE increased significantly already 24 h after therapeutic application of 131I in this group. In patients with small remnants (V < 2.5 ml) a significant but only moderate increase of UIE could be observed (average increase: 47 microg I/g crea). In patients with larger remnants, with relapses or metastases increase of UIE values was significant and more pronounced.


CONCLUSIONS
It was confirmed that UIE increased significantly during radioiodine therapy in patients with DTC and radioiodine-accumulating tissue. The increase of UIE after therapeutic administration of radioiodine can be explained by the disintegrated thyroid follicles in thyroid remnants. The radioiodine-induced iodine release may be one reason for thyroid "stunning" even after application of diagnostic amounts of 131I.
Meller
B
B
University of Lübeck, Department of Radiotherapy and Nuclear Medicine, Ratzeburger Allee 160, 23538 Lübeck, Germany. meller@nuk.uni-luebeck.de
von Hof
K
K
Genina
E
E
Deisting
W
W
Meller
J
J
Richter
E
E
Baehre
M
M
D003404:Creatinine / Q000378:metabolism; D005215:Fasting; D006801:Humans; D007457:Iodine Radioisotopes / Q000493:pharmacokinetics / Q000627:therapeutic use* / Q000652:urine; D008657:Metabolic Clearance Rate; D011859:Radiography; D012016:Reference Values; D013964:Thyroid Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy* / Q000652:urine; D014018:Tissue Distribution; D014055:Tomography, Emission-Computed
D007457:Iodine Radioisotopes; D003404:Creatinine