Dose selection for radioiodine therapy of borderline hyperthyroid patients with multifocal and disseminated autonomy on the basis of 99mTc-pertechnetate thyroid uptake

Abstract

The aim of this study was to optimise radioiodine therapy of diffuse and nodular toxic goitre by calculation of the radiation dose delivered to the thyroid on the basis of the pretreatment technetium-99m pertechnetate thyroid uptake under thyrotropin suppression (TcTU(s)). The TcTU(s) value serves as a substitute for the non-suppressible iodine turnover and the functional autonomous mass. Marinelli's formula was used to calculate tissue absorbed doses of 150 Gy, 200 Gy, 250 Gy and 300 Gy to the thyroids of 438 patients with multifocal and disseminated autonomy. The mean age of patients was 70+/-9 years, and the mean thyroid volume was 54+/-26 ml. Two hundred and sixty-one of the patients had at least one documented previous episode of overt hyperthyroidism. Tissue absorbed doses were adapted to the pretreatment TcTU(s): 150 Gy for a TcTU(s) of 1.5%-2.49%, 200 Gy for a TcTU(s) of 2.5%-3.49%, 250 Gy for a TcTU(s) of 3.5%-4.49% and 300 Gy for a TcTU(s) of > or =4.5%. Normalisation of TcTU(s) and thyrotropin (TSH), thyroid volume reduction and frequency of hypothyroidism and recurrent hyperthyroidism were evaluated 1 year after a single radioiodine therapy. The presented dose strategy resulted in normalisation of TcTU(s) in 96% and an increase in TSH to the normal range in 92%. Recurrent hyperthyroidism was observed in only five patients. Thyroid volume decreased from 54+/-26 before treatment to 34+/-20 ml, a mean reduction of 37%. The frequency of hypothyroidism, at 0.9%, was encouragingly low. Dose selection in accordance with pretreatment TcTU(s) can be recommended for elimination of functional autonomous tissue with a single radioiodine therapy in patients of advanced age with enlarged thyroid glands and relevant autonomous masses who are at risk of developing iodine-induced hyperthyroidism.

Normal reference values for thyroid uptake of technetium-99m pertechnetate for the Namibian population

“…However, the continuous intake of iodised table salt could have reduced thyroid uptake of technetium-99m pertechnetate. This notion is supported by the fact that thyroid uptake of iodine directly correlates with technetium-99m pertechnetate uptake, 18 implying that the mode by which the two radiochemicals are taken up by the thyroid is similar. In an earlier study, Reinhardt et al 19 demonstrated that thyroid uptake of pertechnetate inversely correlated with iodine intake, suggesting that preloading the thyroid with cold iodine can significantly reduce the subsequent uptake of technetium-99m pertechnetate.…”

Development of hypothyroidism during long‐term follow‐up of patients with toxic nodular goitre after radioiodine therapy

“…There are several possible explanations for and factors contributing to the higher incidence rate of hypothyroidism in our patients: First, the higher rate might be due to the longer follow‐up time in our study. When observation is limited to 12 months or less, as in several previous reports, 17,20,27 the overall hypothyroidism incidence rate of hypothyroidism may be lowered, as it increases with ongoing follow‐up time 15,19,21,25 . Second, pretreatment with antithyroid drugs is a possible confounder, because in case of stimulation of a previously suppressed TSH by antithyroid drugs, the risk of hypothyroidism may increase.…”

“…Third, hypothyroidism was defined as elevation of TSH above the normal range, including patients with subclinical hypothyroidism. Other previous studies failed to clearly define the criteria for hypothyroidism 17 or left patients with subclinical hypothyroidism untreated 20 …”

“…The recent German guidelines for radioiodine therapy recommend an absorbed radiation dose of 150 Gy, 16 whereas the European Association for Nuclear Medicine (EANM) guidelines recommend using an intended dose of 100–150 Gy in patients with toxic nodular goitre 4 . Studies with an intended dose of 150 Gy report a success rate of more than 90% 17,18 . However, the incidence of hypothyroidism after radioiodine treatment remains unclear, several studies reporting different incidence rates of hypothyroidism 15,17,19–21 .…”

Toxic Nodular Goiter

“…Consequently, it may be prudent to use temporary antithyroid drug treatment to achieve euthyroidism, discontinue it for several days before and after 131 I administration, and then resume therapy to maintain normal thyroid function while waiting for the effect of the radioiodine, particularly in older patients and those with cardiac disease. 138,139 The incidence of postablative hypothyroidism after radioiodine therapy has been reported to be 25% to 50%, which is lower than that encountered after treatment of patients with Graves' disease. [116][117][118][119][120][121][122][123] One randomized controlled trial has also confirmed this effect of PTU in toxic multinodular goiter.…”