Thyrotoxcosis; Thyroid storm; Subacute thyroiditis; This literature case was received on 13-Nov-2023 and concerned a 30-year-old female patient.   This case report describes a rare case of severe thyrotoxicosis in a woman due to subacute thyroiditis that developed after receiving an influenza vaccine.  The patient had no known past medical history. The patient family history was negative for autoimmune disease. The patient's concomitant medications were not reported.  On an unspecified date, the patient was vaccinated with influenza vaccine (brand not specified; dose, route of administration, anatomical location, and indication: not reported). The batch number was not reported.  On an unspecified date, one day after receiving influenza vaccine, the patient experienced sore throat, odynophagia, and anterior neck pain.  On an unspecified date, the patient experienced other symptoms loss of appetite, chills, fever, fatigue, malaise, abdominal pain, diarrhea, palpitations, heat intolerance and 5lbs weight loss.   On an unspecified date, two weeks after receiving influenza vaccine, the patient visited primary care provider (PCP). Primary care provider for suspected pharyngitis with a course of corticosteroids and unspecified antibiotics.  On an unspecified date, four weeks after receiving influenza vaccine, the patient's symptoms were worsening so the patient was referred to the emergency room. The patient presented to the emergency department (ED) with worsening sore throat, odynophagia, and anterior neck pain. The patient's other symptoms included loss of appetite, chills, fever, fatigue, malaise, abdominal pain, diarrhea, palpitations, heat intolerance, and 5 lbs weight loss. The patient was treated for suspected pharyngitis with a course of unspecified corticosteroids and antibiotics. On exam, the patient appeared anxious and was tachycardic (124 beats per minute) and tachypneic (28 breaths per minute). The patient had no lid lag, stare, thyromegaly or thyroid bruit. However, there was significant tenderness on palpation of the anterior neck. Laboratory evaluation was notable for thyroid stimulating hormone (TSH) <0.01 uIU/mL (0.39 - 4.0.8), free T4 5.19 ng/dL (0.58 - 1.64), free T3 10 pg/ml (2.53-3.87), erythrocyte sedimentation rate (ESR) 95 mm/hr (0-20), and C-reactive protein (CRP) 9.339 mg/dl (0.0-0.9) consistent with thyrotoxicosis. Given Burch-Wartofsky score of 45, there was a concern for impending thyroid storm. The patient received hydrocortisone, methimazole and beta blockers and was admitted to the intensive care unit. The patient responded dramatically to treatment and was discharged within 24 hours on prednisone and metoprolol. On an unspecified date, two weeks post-admission, the 24-hour radioactive iodine uptake (RAIU) test was 2.7 %, consistent with a diagnosis of subacute thyroiditis. A thyroid-stimulating immunoglobulin test (TSI) was negative. The patient continued beta-blockers with a steroid taper for eight weeks.   On an unspecified date, the patient recovered from the events of 'thyrotoxicosis', 'thyroid storm', and 'subacute thyroiditis'. As reported, thyroid function tests and inflammatory markers normalized within three months.  The reporter did not provide a causality assessment. It was reported that subacute thyroiditis was very uncommon after influenza vaccine, and there have been only four reported cases.   The events of 'thyrotoxicosis', and 'thyroid storm' were considered to be serious due to seriousness criteria of medically significant by a Physician within CSL Seqirus's Pharmacovigilance Department.  In conclusion, this case underscores the importance of thoroughly investigating the etiology of severe thyrotoxicosis, given the management and prognosis varies depending on underlying cause. Thyroiditis should be considering in the differential diagnosis of thyroid storm in patients who do not have a personal or family history of autoimmunity and present with neck tenderness in the setting of a precipitating event.  The literature citation Company comment: A 30-year-old patient experienced sore throat, odynophagia, and anterior neck pain one day after receiving the suspect product influenza vaccine. Afterwards, the patient also experienced loss of appetite, chills, fever, fatigue, malaise, abdominal pain, diarrhea, palpitations, heat intolerance and weight loss. The patient was presented to the emergency department four weeks after vaccination. On exam, the patient appeared anxious and was tachycardic, tachypneic, and developed significant tenderness on palpation of the anterior neck. Reportedly, laboratory findings showed elevation of thyroid stimulating hormone, free T3 and T4, erythrocyte sedimentation rate, and C-reactive protein consistent with thyrotoxicosis. Given Burch-Wartofsky score of 45, there was a concern for impending thyroid storm. The patient underwent 24-hour radioactive iodine uptake test, two weeks post-admission, and it was 2.7%, consistent with a diagnosis of subacute thyroiditis. Based on available information causality between the suspect vaccine and all events (subacute thyroiditis, thyroid storm, and thyrotoxicosis) is assessed as possibly related.; Sender's Comments: A 30-year-old patient experienced sore throat, odynophagia, and anterior neck pain one day after receiving the suspect product influenza vaccine. Afterwards, the patient also experienced loss of appetite, chills, fever, fatigue, malaise, abdominal pain, diarrhea, palpitations, heat intolerance and weight loss. The patient was presented to the emergency department four weeks after vaccination. On exam, the patient appeared anxious and was tachycardic, tachypneic, and developed significant tenderness on palpation of the anterior neck. Reportedly, laboratory findings showed elevation of thyroid stimulating hormone, free T3 and T4, erythrocyte sedimentation rate, and C-reactive protein consistent with thyrotoxicosis. Given Burch-Wartofsky score of 45, there was a concern for impending thyroid storm. The patient underwent 24-hour radioactive iodine uptake test, two weeks post-admission, and it was 2.7%, consistent with a diagnosis of subacute thyroiditis. Based on available information causality between the suspect vaccine and all events (subacute thyroiditis, thyroid storm, and thyrotoxicosis) is assessed as possibly related.