SAT-586 Primary Fallopian Tube Carcinoma Metastasizing to the Thyroid Gland: A Rare Cause of Hypothyroidism

Background: Primary fallopian tube carcinoma (PFTC) has been considered to account for only about 0.1-0.5% of all gynecologic cancers. It is notorious for metastasis to the ovaries, omentum, and uterus but has never been reported to present with hypothyroidism as a metastatic thyroid nodule. Case: A 53-year-old female presented with fatigue, generalized weakness and constipation for the last 4 months. Her past medical history was significant for stage IV poorly differentiated PFTC status post total abdominal hysterectomy, omentectomy and tumor debulking due to the local spread of the PFTC. She also underwent multiple cycles of chemotherapy with paclitaxel, cisplatin, doxorubicin and bevacizumab and was declared in remission by her oncologist. On examination, her vital signs were stable. Thyroid examination showed palpable nodules in the right and left lobes. Her laboratory investigation revealed normal liver and renal function tests, but the blood chemistry was significant for raised TSH levels of 68.8 mcIU/ml and decreased T3 and T4 levels of 45 ng/dL and 0.54 ng/dL respectively. Thyroid Peroxidase Antibody level was less than 3 IU/ml (reference range <6 IU/ML). Thyroid ultrasound showed a 2.3 x 0.7 x 2 cm solid nodule in the medial lower pole of the right lobe with and a 1.9 x 0.8 x 1.1 cm solid nodule in the medial lower pole of left lobe with increased vascularity. The fine needle aspiration cytology revealed groups of highly atypical cells showing enlarged, irregular, crowded nuclei which were intermixed with scattered bland follicular cells and macrophages. These atypical cells were identical to the serous carcinoma cells seen on previous PFTC specimens, confirming the hematogenous spread of PFTC to the thyroid gland. This diagnosis was further substantiated by immunohistochemical staining, which showed two different populations of cells, thyroid cells stained positive for TTF-1 (Thyroid transcription factor-1) while other cells were positive for WT-1 immunohistochemical stain strongly suggestive of metastatic serous carcinoma. Her serum cancer antigen 125 (CA 125) was also found to be elevated at 395 U/ml (normal range 0-35 U/ML) suggesting recurrence of PFTC. Niraparib chemotherapy was initiated. She was also started on 75 mcg of Levothyroxine per day. TSH levels after two months were 4.2 mcIU/ml. Conclusion: PFTC can be diagnosed early in its course using transvaginal Doppler ultrasound to detect areas of neovascularization within the fallopian tube. PFTC has histological and clinical resemblance with ovarian and uterine primaries and hence can be challenging to diagnose. It is incredibly uncommon to suspect thyroid metastasis from the residuals of PFTC as a possible etiology of hypothyroidism. Thus, physicians should keep a very high index of suspicion in appropriate clinical settings. Prognosis remains poor as most patients present with metastatic disease.