ODP456 A rare case of massive goiter with overlapping features of Riedel's thyroiditis and Fibrous Variant of Hashimoto's thyroiditis

BACKGROUND: Hashimoto's thyroiditis is the most common autoimmune disorder of the thyroid gland associated with hypothyroidism. It is distinguished by painless inflammation and elevated titers of TPO antibodies. Fibrous Variant of Hashimoto's Thyroiditis is a rare condition occurring in about 10% of cases, mainly middle-aged individuals, and characterized by an extensive fibrous proliferation without extension into the surrounding structures. Riedel's thyroiditis is a rare generalized autoimmune disorder involving B lymphocytes and IgG4-secreting plasma cells, clinically characterized by extension of the fibrous process to surrounding tissues in the neck, as well as compression symptoms like dysphagia and dyspnea. It is usually associated with mediastinal and retroperitoneal fibrosis. There is significant clinical, pathological, and laboratory test overlap between these two entities that makes the differential diagnosis difficult. CASE REPORT: A 54-year-old male diagnosed with hypothyroidism on Levothyroxine, and a large goiter seen with new onset of compressive symptoms including dyspnea and dysphagia since March/April 2021. He complains of having pressure on his neck, difficulty swallowing, and worse symptoms when lying flat on a bed. His thyroid is noted to be extremely large and firm, without evident nodularity on examination. Laboratory exams were significant for TSH 3.21 (0.36-3.74), T4 0.94 (0.76-1.46), vitamin D level 13 (30-100), TPO > 9000 (0-9), IgG4 81.9 (4. 0-86. 0), and a negative ANA screen. Thyroid ultrasound showed an enlarged heterogeneous thyroid gland with no discrete nodules. Given the compressing symptoms the patient presented, he was referred to surgery for total thyroidectomy. Surgery could not be completed due to the inability to safely mobilize the gland and adherence to surrounding tissues. He then underwent a right thyroid lobule core biopsy, showing an inflammatory infiltrate composed primarily of lymphocytes and plasma cells. Atrophic thyroid follicles with prominent fibrosis were noted. IgG4 immunostaining highlights <5 positive plasma cells/HPF and IgG4/IgG <30%, excluding IgG4-related fibrosing disease in this core biopsy material. There was no evidence of malignancy. Considering the prominent fibrosis, very high TPO Ab titer, and a seeming lack of extrathyroidal involvement, Fibrous Variant of Hashimoto's Thyroiditis seems more likely, but Riedel Thyroiditis cannot be completely ruled out. He was placed on Prednisone 40 mg daily, with modest improvement of symptoms. Tamoxifen 20mg daily was later added given literature reports of additional improvement in symptoms associated with its use. Referral to a tertiary center for possible isthmusectomy may be considered if symptoms worsen. Presentation: No date and time listed