[Reduced radioiodine uptake at increased iodine intake and 131I-induced release of "cold" iodine stored in the thyroid].
OBJECTIVE
The extent of urinary iodine excretion (UIE) provides information about iodine supply and release. In the present study we investigated correlations between UIE and radioiodine uptake (RIU) as well as effects of radioiodine therapy on UIE in patients with autonomous goitre.


METHODS
In 197 consecutive patients with thyroid autonomy, UIE was measured twice during radioiodine test (RITe) and correlated with RIU. In 98 of these patients, thyroglobulin and thyroid volume (V) were determined prior to therapy. Individual changes in urinary iodine excretion (DeltaUIE) and TG (DeltaTG) could be investigated four weeks (4W) and six months (6M) after radioiodine therapy. Additionally, DeltaV was determined 6M after therapy. DeltaUIE, DeltaTG and DeltaV were correlated with target dose and target volume.


RESULTS
Patients with higher iodine excretion exhibited significantly lower thyroidal radioiodine uptake values. Twofold increased UIE prior to therapy decreased radioiodine uptake by 25%. Compared with pretherapeutic values, UIE and TG were significantly increased four weeks after radioiodine therapy (p < 0.001). Median values of both parameters were found to be doubled. The product of target dose and target volume was not only correlated with a decrease of thyroid volume 6M after therapy, but also with an increase of UIE and TG in the early phase after therapy.


CONCLUSIONS
It was confirmed that UIE during RITe is a measure for iodine intake and can be used to investigate the competition between stable iodine and radioiodine. The increase of UIE and TG four weeks after therapeutic administration of radioiodine can be explained by disintegrated thyroid follicles. The therapy-induced iodine release may be one important cause for the development of hyperthyroidism in some patients during the first weeks after radioiodine therapy. It may contribute to the known decrease of radioiodine uptake after preapplications of 131I in various thyroid diseases.
Meller
B
B
University of Luebeck, Department of Radiotherapy and Nuclear Medicine, Ratzeburger Allee 160, 23538 Lübeck, Germany. meller@nuk.uni-luebeck.de
Haase
A
A
Seyfarth
M
M
Wenzel
B E
BE
Richter
E
E
Baehre
M
M
D006801:Humans; D007455:Iodine / Q000652:urine*; D007457:Iodine Radioisotopes / Q000493:pharmacokinetics*; D013954:Thyroglobulin / Q000378:metabolism; D013961:Thyroid Gland / Q000187:drug effects / Q000378:metabolism*; D013974:Thyroxine / Q000494:pharmacology
D007457:Iodine Radioisotopes; D013954:Thyroglobulin; D007455:Iodine; D013974:Thyroxine