SUN-304 - A Case of Myxedema Coma in A Patient with Systemic Amyloidosis

Introduction: The most common cause of hypothyroidism in iodine-sufficient regions is autoimmune thyroid disease. Less commonly, patients with amyloidosis may develop thyroid gland infiltrates leading to hypothyroidism in addition to more commonly affected organs such as the kidneys, liver, and heart. Some studies report thyroid infiltration in roughly 15–50% of patients with AL amyloidosis and 20–80% of those with AA amyloidosis; however, only a small subset develops clinically overt hypothyroidism (approximately 7%). We report a case of newly diagnosed, seronegative hypothyroidism presenting as myxedema coma in a patient with newly diagnosed amyloidosis.

Case presentation: A 65-year-old man with a medical history of chronic kidney disease, congestive heart failure, and atrial fibrillation presented with anasarca. He had been diagnosed with hypothyroidism on outpatient laboratory testing four months prior but was nonadherent to medication. He had no family history of hypothyroidism and no history of medication use associated with thyroid dysfunction, iodine exposure, or prior neck radiation. Admission labs showed TSH >46 µIU/mL, FT4 < 0.5 ng/dL, and negative thyroid peroxidase antibodies. The patient was somnolent with slowed responsiveness, hypothermic (temperature 35.1°C - 37.1°C), and bradycardic (heart rate 40–60 beats per minute) with a wide QRS complex on electrocardiogram. Additional findings included diffuse lymphadenopathy, anasarca with pulmonary edema and ascites, and acute renal failure. Further evaluation with cardiac MRI demonstrated infiltrative cardiomyopathy. Abdominal fat pad and lymph node biopsies were positive for Congo red staining, with further subtyping showing AA amyloidosis. Thyroid ultrasound showed a normal sized thyroid gland without nodules, heterogeneity, or focal lesions. The patient was treated with intravenous levothyroxine and hydrocortisone for suspected myxedema coma, resulting in normalization of FT4 levels and clinical improvement.

Discussion: Amyloid deposits in the thyroid gland have been reported in ~30–80% of patients with systemic amyloidosis in autopsy or histopathology studies. Several reported cases describe amyloid infiltration of the thyroid without associated glandular enlargement. Although our patient had no radiographic evidence of overt amyloid goiter or fine-needle aspiration to confirm involvement, the recent onset of seronegative overt hypothyroidism coinciding with severe systemic amyloidosis raises concern for microscopic amyloid infiltration of the thyroid gland as a potential mechanism of hypothyroidism.

Conclusion: Overt hypothyroidism in primary amyloidosis is rare. This case highlights the need for routine thyroid function tests in patients with systemic amyloidosis and underscores the importance of prompt treatment to avoid delays in managing hypothyroidism.

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