To the Editors:
Royal et al
1.
recently published an interesting article in Surgery on the potential role of thyroid hormonal suppression in patients with primary hyperparathyroidism (PHPT). In their series of 99 patients with PHPT, the authors reported only a 70% sensitivity of sestamibi scintigraphy by use of planar and tomographic (SPECT) imaging without thyroid suppression, somewhat lower than in other studies reporting a sensitivity of 90% or more with sestamibi SPECT.2.
, 3.
Their purpose is to improve sensitivity of technetium 99m–sestamibi scintigraphy in localizing parathyroid adenomas so that a higher percentage of patients with PHPT might benefit from minimally invasive parathyroidectomy. The authors hypothesized that the suppression of thyroid metabolism after thyroid hormone therapy could decrease sestamibi thyroid uptake, thus improving parathyroid adenoma visualization. In their exprience thyroid hormone suppression (either with l-thyroxine 100 μg daily for 4 weeks or liothyronine 25 μg 3 times a day for 10 days) allowed a successful sestamibi parathyroid localization in 10 of 14 patients (71% of cases) after an initial nonlocalizing sestamibi scintigraphy.It should be noted that sestamibi is currently used in the follow-up of patients with differentiated thyroid cancer to early detect recurrent disease without the discontinuation of thyroid hormone therapy. In our experience
4.
normal thyroid remnants were visualized by sestamibi, as well as metastases in patients with thyroid cancer who have undergone total thyroidectomy and thyroid hormone therapy. The difference in the radiotracer uptake intensity between normal thyroid remnants and metastatic deposits was not significant.4.
We would therefore assume that the thyroid-stimulating hormone effect would not be expected to have an important influence on sestamibi updtake by thyroid tissue.A standard dose for planar and SPECT parathyroid scintigraphy is 740 MBq, and the labeling efficiency of sestamibi with 99mTc-pertechnate is ususally 90% to 95%. It can be calculated that approximately 37 to 74 MBq of the given dose is nonlabeled 99mTc-pertechnetate, equivalent to the dose of this radiotracer used for thyroid scintigraphy. Avidly trapped by normal thyroid tissue, 99mTc-pertecnetate washes out very slowly from the normal thyroid tissue in comparison to sestamibi. Thus the relatively high uptake of 99mTc-pertechnetate by the thyroid might actually interfere with the visualization of a sestamibi-avid parathyroid adenoma, especially one located behind or close to the thyroid contour.
3.
This effect might be contributed to some of the initially nonlocalizing scintigraphic examinations in Royal's study.The authors suggested that the improvement in parathyroid visualization they achieved by use of thyroid hormonal suppression was related to a decrease in thyroid sestamibi uptake. We are suggesting that the improvement could be due instead to blocking thyroid uptake by nonlabeled 99mTc-pertechnetate. If so, the benefit could be more readily achieved by the oral aministration of a single 400-mg tablet of potassium perchlorate, a powerful antagonist of 99mTc-pertechnetate thyroid uptake and retention.
3.
This simple expedient might avoid the 4-week protocol of thyroid hormone treatment. It seems unlikely, however, that thyroid suppression could greatly improve identification of parathyroid adenomas in patients with concomitant sestamibi-avid thyroid nodules because thyroid hormone will not reduce radiotracer uptake in such nodules.4.
When a minimally invasive parathyroidectomy is being considered, the most sensitive scintigraphic technique available should be used. Either sestamibi SPECT or double-tracer scintigraphy (with the iodine 123/sestamibi or 99mTc-pertechnetate/sestamibi technique) has been at least 90% sensitive in localizing solitary parathyroid adenomas.
2.
, 3.
We doubt that thyroid hormone suppression will yield further improvement.References
- Improving the yield of preoperative parathyroid localization: technetium Tc 99m-sestamibi imaging after thyroid suppression.Surgery. 2002; 132: 968-975
- Concise parathyroidectomy: the impact of preoperative SPECT 99mTc sestamibi scanning and intraoperative quick parathormone assay.Surgery. 1997; 122: 1107-1116
- Role of gamma probes in performing minimally invasive parathyroidectomy in patients with primary hyperparathyroidism: optimization of preoperative and intraoperative procedures.Eur J Endocrinol. 2003; 149: 7-15
- The role of technetium-99m-methoxyisobutylisonitrile scintigraphy in the planning of therapy and follow-up of patients with differentiated thyroid carcinoma.Eur J Nucl Med. 2000; 27: 431-440
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© 2004 Elsevier Inc. Published by Elsevier Inc. All rights reserved.
