Amyloid goitre in a patient with thyrotoxicosis: a diagnostic challenge

Amyloid goitre is an uncommon cause of thyroid enlargement resulting from substantial amyloid deposition within the gland, sufficient to cause a clinically detectable goitre. Most affected patients are euthyroid. Amyloid deposition within the thyroid gland is typically detected incidentally or in association with thyroid malignancies, such as medullary thyroid carcinoma. The occurrence of a clinically detectable goitre secondary to amyloid deposition in a patient with concomitant thyrotoxicosis is exceptionally rare. We describe a case of an 81-year-old woman who presented with a multinodular goitre and biochemical thyrotoxicosis with suppressed TSH, elevated free T4, and negative thyroid antibodies (including TSH receptor antibodies). An ultrasound of the thyroid gland demonstrated a suspicious TI-RADS 3 nodule, and thyroid scintigraphy showed an enlarged right lobe with a large cold nodule in the inferior two-thirds of the right lobe with inhomogeneous uptake. Histopathology demonstrated extensive amyloid deposition within the nodule, confirming a diagnosis of amyloid goitre/amyloidoma. Systemic evaluation identified a monoclonal IgG kappa band on serum protein electrophoresis and bone marrow plasma cell infiltration of less than 10%, consistent with a monoclonal gammopathy of undetermined significance (MGUS). No clinical or investigative evidence of systemic amyloidosis was identified.