MON-385 Bone in Thyroid: An Unusual Case of Osseous Metaplasia of the Thyroid

Abstract Disclosure: A.K. Fernando: None. M. Ahmad: None. J. Melendez-Rivera: None. Background: Thyroid nodules are a common clinical entity, and its prevalence is estimated to be 5% in women and 1% in men living in iodine-sufficient parts of the world. These nodules may undergo a wide range of degenerative changes such as infarction, hemorrhage, fibrosis and dystrophic calcification. Osseous metaplasia with mature bone formation is a rare occurrence. We present a case of a thyroid nodule proven to have mature bone formation. Case: A 50-year-old African American female with a history of a thyroid cyst and a motor vehicle accident 1 year prior was referred to endocrinology for evaluation of a large cystic thyroid mass found on an MRI. She recalls having multiple simple aspirations of her thyroid cyst during childhood. She is a non-smoker, has had no exposure to radiation and has no family history of thyroid cancer. Upon questioning, she expressed some discomfort in the left side of her neck while eating and yawning. On physical exam, the left lobe of the thyroid was enlarged with a palpable nodule. TSH and free T4 were within normal levels. A thyroid ultrasound showed a 4.2 x 3.6 x 2.9 cm mixed cystic and solid hypoechoic nodule with peripheral calcifications and lobulated margins. This nodule was scored a TIRADS 5. An FNA was attempted, however was non diagnostic. CT neck re-demonstrated the left thyroid nodule causing mass effect on the trachea with tracheal deviation. She was referred to endocrine surgery and underwent a left thyroid lobectomy. On microscopic examination, nodular hyperplasia with focal fibrosis was noted in addition to heterotopic bone with fatty marrow. Discussion: Metaplasia is a form of cellular adaptation to an external stressor that exerts deleterious effects on cellular function. It poses the question whether osseous metaplasia in the thyroid seen in our patient could be related to the trauma from recurrent aspirations during childhood or the recent motor vehicle accident. The pathophysiology of this clinical entity is not well understood. Several studies have proposed bone morphogenic proteins to play a significant role in ectopic bone development in the thyroid gland. There are reports of osseous metaplasia in the thyroid coexisting with hematological malignancies which raises the need to screen for such malignancies. On the other hand, the risk of thyroid cancer associated with this phenomenon is not well understood. We suggest further research to determine the effects of these metaplastic changes on the risk of developing thyroid cancer. Presentation: Monday, July 14, 2025