Subacute (de Quervain) Thyroiditis Evolving Into Permanent Hypothyroidism in a Patient With Kikuchi-Fujimoto Disease: A Case Report With Four-Year Follow-Up

Subacute (de Quervain) thyroiditis is a painful, usually self-limiting inflammation of the thyroid that progresses from thyrotoxicosis to recovery.Kikuchi-Fujimoto disease (KFD) is a rare, benign necrotizing lymphadenitis.Both conditions involve cytotoxic T-cell injury and specific human leukocyte antigen (HLA) associations, yet their coexistence is rarely reported.We describe a 43-year-old woman with biopsy-proven KFD on maintenance colchicine who was started on hydroxychloroquine in August 2022 for suspected KFD activity and later that month developed fever, anterior neck pain with marked thyroid tenderness, dysphagia, palpitations, and weight loss.Thyroid-stimulating hormone (TSH) was 0.012 µIU/mL, free thyroxine 2.76 ng/dL, thyroglobulin 721 ng/mL, and erythrocyte sedimentation rate 91 mm/h; thyroid autoantibodies were negative, and Doppler ultrasonography showed an enlarged, hypovascular gland.After treatment with prednisone and atenolol, she entered symptomatic hypothyroidism (peak TSH 10.6 µIU/mL) and began levothyroxine 25 µg/day.Withdrawal of levothyroxine was planned in 2025 but not undertaken because hypothyroid symptoms recurred, and the dose was increased to 50 µg/day.Four years after onset, she remains euthyroid on levothyroxine replacement and without serological evidence of systemic lupus erythematosus.Thyroid autoantibodies were negative at the onset of thyroiditis.This case documents an uncommon overlap of the two diseases and shows that subacute thyroiditis may be followed by persistent hypothyroidism despite glucocorticoid treatment and negative thyroid autoantibodies at onset, supporting long-term surveillance of thyroid function.