MON-450 A Case of Large Inflammatory Pseudonodules Secondary to Subacute Thyroiditis

Abstract Disclosure: M.A. Lim: None. M.H. Shanik: None. Background: Subacute thyroiditis is a relatively rare but self-limiting thyroid disorder. It progresses through three phases: an initial thyrotoxic phase, followed by the hypothyroid phase, and ultimately a euthyroid phase. In rare cases, it has been associated with the development of thyroid nodules which typically regress after thyroiditis resolves, leading to the term “inflammatory pseudonodules". Case: A 68-year-old male with no significant PMH presented with a three-week history of low-grade fever, night sweats, tremors, palpitations, and a 10-lb weight loss. Workup for infection was negative, and he was diagnosed with fever of unknown origin. Examination revealed a mildly tender, enlarged thyroid with a nodular appearance and a fine tremor of the hands. Thyroid function tests confirmed thyrotoxicosis: TSH 0.01 (0.4-4.0) mIU/L, Total T4 17.3 (4.5-12) mcg/dL, FT4 7.1 (1.4-3.5) ng/dL, and T3 uptake 41% (22-35). Inflammatory markers were elevated (ESR 85 mm/hr and hsCRP 9.9 mg/dL), while anti-thyroglobulin and anti-TPO antibodies were negative. Ultrasound showed a 3.3 cm hypoechoic right mid-lobe nodule and a 4 cm left middle-lobe nodule, both with smooth margins and no malignant features. Methimazole was empirically started pending further work-up. Thyroid uptake and scan eventually showed a dominant warm nodule at the right upper pole, with 24-hour radioactive iodine uptake at 5.2%. Fine-needle aspiration biopsy revealed Bethesda Category II (benign findings). At two-week follow-up, thyroid function tests consistent with hypothyroidism (TSH 62.77 mIU/L, FT4 0.31 ng/dL, Total T3 0.77 ng/mL), methimazole was discontinued and levothyroxine was initiated. Three months later, he achieved a euthyroid state, and a repeat ultrasound showed no nodules. Subsequent ultrasounds remained normal, confirming complete regression of the nodules. Discussion: Subacute thyroiditis has an annual incidence of 12.1 cases per 100,000 and is primarily attributed to viral or post-viral inflammatory process. Patients typically present with tender thyroid gland, fever, malaise, and fatigue. Elevated ESR/CRP and low radioiodine uptake during thyrotoxicosis confirm the diagnosis. Nodules, when present, are thought to result from focal dense inflammatory infiltration and show characteristic ultrasound features: marked hypoechogenicity, smooth margins, ovoid shape, and absence of calcifications. A study by Hepsen et al. (2002) found that 28.6% of patients with subacute thyroiditis had nodules at diagnosis, with large nodules (≥4 cm) being rare. Regardless of size, pseudonodules resolve spontaneously as inflammation subsides. Conclusion: Inflammatory pseudonodules may mimic true thyroid nodules on ultrasound, but these are transient findings resulting from inflammation rather than true neoplasms. Treatment is conservative and pseudonodules typically resolve completely over time. Presentation: Monday, July 14, 2025