7205 Anaplastic Thyroid Carcinoma Presenting as New-Onset Thyrotoxicosis and Hypoparathyroidism in a Young Patient: A Diagnostic Dilemma between Hashitoxicosis and Anaplastic Thyroid Cancer

Abstract Disclosure: M. Sulehri: None. E.G. Bustamante: None. A. Elmaaz: None. A. Chaudhuri: None. Introduction: This case report explores the complex diagnostic challenges encountered in a young patient presenting with anaplastic thyroid carcinoma (ATC) manifested by thyrotoxicosis and hypoparathyroidism. Case report: A 29-year-old male who sought multiple emergency room visits for shortness of breath, was initially misdiagnosed with pneumonia. On the third visit, exacerbated respiratory distress led to intubation, CT of the neck and thyroid US unveiling an infiltrative mass centered within the region of the thyroid gland. Initial thyroid function tests showed TSH <0.02 uIU/mL, free thyroxine 6.91 ng/dL, and total T3 204 ng/dL indicating hyperthyroidism, promptly managed with Methimazole and beta blockade. Elevated TPO antibodies (194 unit/mL) were observed, whereas thyroglobulin antibodies and thyroid receptor antibodies were undetected. Concurrently, hypoparathyroidism was noted, marked by low calcium levels (7.9 mg/dL) with albumin of 3.4 g/dL (corrected Calcium 8.4), and low parathyroid hormone (PTH) (<6 pg/mL). Other labs showed ionized calcium 4.30 mg/dL, phosphate 4.6 mg/dL, magnesium 1.8 mg/dL, and vitamin D 54 ng/dl. The patient received calcium carbonate supplement, calcitriol and IV calcium gluconate. Thyrotoxicosis then transitioned to hypothyroidism and levothyroxine replacement therapy was initiated. Definitive diagnosis was delayed due to the patient's overall condition and a low suspicion of malignancy. Left thyroidectomy and tracheostomy were performed, revealing dense subplatysmal necrosis with tumor involvement of both thyroid lobes and isthmus with multiple enlarged nodes. Final surgical pathology confirmed the presence of anaplastic thyroid carcinoma. Due to poor prognosis, the patient was transferred to hospice care where the patient succumbed to his condition within a day. Conclusion: Anaplastic thyroid cancer is considered one the most aggressive and fastest growing tumors. Although it is usually seen in patients after the age of 60, we present a very rare case of ATC in a young patient in the third decade who was presented with infiltrative thyroiditis and hypoparathyroidism. The case underscores the importance of considering rare etiologies in patients with atypical clinical presentation. Presentation: 6/1/2024