8141 A Rare Case of Follicular Adenoma with Osseous Metaplasia and Mature Bone Formation- Is Surveillance Important?

Abstract Disclosure: K. Madani: None. Y. Al-Khazraji: None. D.H. Sacoto: None. G.K. Rai: None. R. Belokovskaya: None. A.A. Franco-Akel, MD: None. Osseous metaplasia and mature bone formation (MBF) are common in malignant thyroid and parathyroid tumors. It is very unusual to have MBF in a benign tumor-like follicular adenoma, as in the case we describe. We found only twelve such cases reported in the literature, and none addressed the cancer risk or need for surveillance for the development of potential malignancy. Thus, further studies are required to investigate the need for surveillance in patients not undergoing excision and in those with remnant thyroid tissue post-removal of the adenoma for recurrence. A 43-year-old female with history of hyperlipidemia and hypothyroidism had an incidental finding of midline neck mass during the evaluation of a left upper extremity lipoma. Ultrasound of the neck showed two nodules in the Left (L) thyroid lobe: a complex nodule measured 2.2 x 1.2 x 2.1 cm and a calcified nodule measuring 1.5 x 0.8 x 0.6 cm. MRI of the neck revealed a 1.9 cm heterogeneously enhancing nodule in the mid to lower pole of the L thyroid lobe. The patient had symptoms suggestive of compression, such as dysphagia, odynophagia, and hoarseness; thus, left hemothyroidectomy was performed. Tissue biopsy was notable for follicular adenoma with osseous metaplasia and mature bone formation. The patient remained clinically and biochemically euthyroid, with only mildly elevated thyrotropin level, and treatment with levothyroxine 25 mcg once daily was initiated. On post-surgical follow-up, there was a lack of data to support close monitoring modality and strategy for the remnant thyroid lobe and neck nodal compartments. We concluded with the patient to follow up in a clinical visit for a physical exam of the neck and to obtain a follow-up sonogram at an interval of six months, in addition to monitoring TSH levels to guide hypothyroidism therapy. Osseous metaplasia has not been established as a premalignant condition, but in certain tissues like the esophagus, metaplasia is a risk factor for malignant conversion, for instance, in Barrett’s esophagus leading to esophageal adenocarcinoma. Moreover, MBF in thyroid tissue has been attributed to the activity of basic fibroblast growth factor (bFGF) and bone morphogenetic protein-2 (BMP-2), which are linked with mitogenesis and tumorigenesis. Thus, the question arises if post-excision surveillance is required in patients with a history of osseous metaplasia in remnant thyroid tissue post-lobectomy or partial thyroidectomy. At this point, more epidemiological data on similar cases may help us understand the true risk of this clinical scenario, in order to develop a reasonable approach for surveillance. Presentation: 6/3/2024