8062 Uncommon Presentation of Multiple Myeloma as a Rapidly Growing Thyroid Goiter Initially Thought to be Hashimoto's Thyroiditis

Abstract Disclosure: M.S. Shah: None. S. Humayon: None. K.P. Sanu: None. C. Peñaherrera: None. We present a unique case of an 85-year-old female with rapidly growing anterior neck swelling and subclinical hyperthyroidism, initially suggestive of Hashimoto's thyroiditis. The goiter initially responded well to steroids but rebounded back in size despite two courses of prednisone. This prompted further diagnostic workup which revealed unexpected findings, ultimately leading to the diagnosis of multiple myeloma involving the thyroid parenchyma and lymph nodes. The patient had no history of thyroid disorders or recent upper respiratory tract infections. She exhibited sudden neck swelling, mild subclinical hyperthyroidism, and positive TPO antibodies. Initial fine needle aspiration suggested lymphoid tissue with Hurthle cell differentiation, raising suspicion of Hashimoto's thyroiditis. Flow cytometry supported this diagnosis, but the potential for a Hurthle cell neoplasm persisted. Prednisone temporarily reduced thyroid size, but disease recurrence necessitated surgical excisional and incisional biopsies. Subsequent biopsies revealed plasma cell neoplasms consistent with multiple myeloma. The pathologic report confirmed MYC rearrangement, indicative of multiple myeloma involvement in lymph nodes and thyroid tissue. Protein electrophoresis revealed two restricted bands migrating in the gamma globin region. The patient experienced subclinical hyperthyroidism that evolved into euthyroidism over a month. Incidental findings included non-invasive, low grade papillary urothelial carcinoma. Disease progression was evident in a follow-up PET/CT, prompting oncology consultation. The patient initiated Revlimid with chemotherapy. Levothyroxine was prescribed for postoperative hypothyroidism, with adjustments based on thyroid panel results. This case report illustrates the intricate diagnostic challenges posed by multiple myeloma masquerading as a thyroid disorder. The unexpected evolution of the disease, marked by MYC rearrangement and metastatic spread, emphasizes the need for heightened vigilance in managing such cases. Clinicians should consider alternative diagnoses when treatment efforts fail to provide adequate response. This will ensure timely and appropriate management in clinical practice. This report contributes valuable insights into the diagnostic complexities and multidisciplinary considerations for addressing similar presentations in endocrine and oncology settings. Presentation: 6/1/2024