8477 Medullary Thyroid Carcinoma Masquerading as a Cold Nodule in a Patient With Graves' Disease

Abstract Disclosure: T. Umashree: None. K.J. Farooqui: None. A. Mithal: None. Background: Only a handful of cases of MTC have been reported in the setting of Graves’ disease. Clinical Case: A 38 year old lady presented with symptoms of significant weight loss and palpitations for past six months. Thyromegaly was present, with non tender left lobe enlargement of about 3x3 cm. On further probing it was found that her father had been operated for thyroid cancer 22 years ago, the type of which patient was not aware of. TFTs reflected a thyroxtoxic state (TSH -<0.01mIU/mL [0.34-5.6] FT4 - 3.38 ng/dL [0.58-1.64] FT3 - 8.46 pg/mL [2.6-4.2]) and TRAb was positive (8.74 IU/L [<1.22]). USG neck revealed a hypoechoic lesion of 18x18x13 mm in left lobe thyroid with internal calcifications, with multiple bilateral non-significant lymph nodes. Technetium99 (Tc99) scan showed an enlarged left lobe, with diffuse thyroid uptake at 20 minutes of 4%, and a well-defined photopenic area in left lobe, corresponding to the nodule seen on USG. FNAC of the nodule was suggestive of Hürthle cell neoplasm and patient underwent left hemithyroidectomy. Surprisingly, HPE was consistent with medullary thyroid cancer. Serum calcitonin was raised at 148 pg/mL (N<4.8 for females) and plasma free metanephrines and normetanephrine were normal. F-18 DOPA PET CT showed uptake in residual right lobe of thyroid only, no other abnormal uptake was seen. She underwent completion thyroidectomy with central compartment clearance and is now under regular follow up. Genetic analysis showed RET gene mutation of Cys634Ser, which is heterozygous for MTC. Cold nodules in Graves’ disease should be viewed with suspicion, and while PTC and FTC have been reported more commonly, MTC is extremely rare in this setting. A calcitonin washout of the FNAC may have prevented this patient from undergoing two surgeries. Moreover, Hürthle cell change can also occur in C-cells of the thyroid, not just the follicular cells. Lastly, given the history of thyroid cancer in her father, index of suspicion of MTC should have been higher in this patient. Conclusion: Cold nodules in Graves’ Disease may be malignant, and should always be examined by FNAC. Though MTC is rare, it is not unknown in Grave’s disease. Presentation: 6/3/2024