Absent radiotracer uptake in thyroid gland in parathyroid scintigraphy with 99mTc-MIBI: A case report

Localization of 99mTc- hexakis-2-methoxyisobutylisonitrile (99mTc-MIBI) by parathyroid adenomas is well known, and this warrants MIBI scan to evaluate suspected parathyroid adenoma in primary hyperparathyroidism. Typically, the radionuclide concentrates in both the thyroid gland and parathyroid adenoma in early images, and later on delayed images washes out slowly from the parathyroid adenomas located in the neck or mediastinum, in comparison with more rapid wash out from the thyroid gland. We report a 71-year old woman with history of hypothyroidism, who has been on levothyroxine therapy for 5 years, and primary hyperparathyroidism, for which a double phase 99mTc-MIBI parathyroid scintigraphy was performed. Although the planar views demonstrated no evidence of radiotracer uptake in thyroid gland, single photon emission computed tomography/computed tomography (SPECT-CT) images revealed the presence of thyroid gland with a multinodular pattern on CT component of the study. Also planar images showed no focal uptake, but in SPECT-CT evaluation a MIBI-avid nodule was depicted in the posteromedial aspect of lower portion of left thyroid lobe, representing a parathyroid adenoma, later confirmed by pathology after surgical resection. The possible explanation for non-visualization of thyroid gland could be thyroid suppression with levothyroxine.