Influence of urinary iodine excretion on thyroid technetium-99m pertechnetate uptake with and without TSH suppression: what happens when iodine supply increases?

This study examines how thyroid pertechnetate uptake with and without thyroid-stimulating hormone (TSH) suppression changes as a function of increasing iodine supply. This is of special interest in countries at the threshold of sufficient iodine supply, where thyroid scintigraphy plays a key role in thyroid examination, especially for the diagnosis of Plummer's disease. From 1995 to 1997, a total of 1069 patients with euthyroid goitre, Plummer's disease or Graves' disease were included in the study. All patients underwent thyroid examination including sonography, scintigraphy with technetium-99m pertechnetate, and determination of free triiodothyronine, free thyroxine, TSH and urinary iodine excretion. Iodine excretion in the range from 0 to 500 microg iodine/g creatinine showed an inverse correlation with thyroid pertechnetate uptake, but no correlation with TSH was observed. There was no correlation between thyroid pertechnetate uptake and iodine excretion when TSH stimulation was eliminated, with two exceptions: thyroid pertechnetate uptake was significantly increased for iodine excretion values below 50 and 100 microg iodine/g creatinine in patients with Graves' and Plummer's disease, respectively. When iodine excretion exceeded 500 microg iodine/g creatinine, pertechnetate uptake was reduced to a basal level independent of the TSH. In conclusion, the influence of TSH on the thyroid pertechnetate uptake seems to be secondary compared with the influence of the iodine supply. It can be concluded further that the reference range of thyroid pertechnetate uptake under TSH suppression will not change significantly when the iodine supply increases from conditions of mild iodine deficiency to iodine sufficiency. Thyroid pertechnetate uptake with and without TSH suppression cannot be reliably interpreted beyond an iodine excretion of 500 microg iodine/g creatinine.