Subacute thyroiditis following an Influenza Vaccine; heat intolerance; worsening sore throat; odynophagia; palpitations; anterior neck pain; subacute thyroiditis following severe thyrotoxicosis; loss of appetite; fatigue; malaise; Thyroid Storm/thyrotoxic crisis due to subacute thyroiditis; tachypneic; chills; fever; abdominal pain; tenderness on palpation of the anterior neck; diarrhea; tachycardic; anxious; Initial information received on 13-Nov-2023 via healthcare Professional regarding an unsolicited valid serious case issued from a literature article:    This case involves a 30-year-old female patient who experienced subacute thyroiditis following severe thyrotoxicosis, thyroid storm/thyrotoxic crisis due to subacute thyroiditis and subacute thyroiditis following an influenza vaccine (Latency: 1 day).  The patient's past medical history, medical treatment(s), vaccination(s) and family history were not provided.  On an unknown date, the patient received a dose of suspect INFLUENZA VACCINE produced by unknown manufacturer lot number not reported via unknown route in unknown administration site.  Patient with no known past medical history presented to the emergency department (ED) with worsening sore throat (oropharyngeal pain), odynophagia and anterior neck pain (neck pain). Symptoms began 4 weeks ago prior to presentation, 1 day after receiving an influenza vaccine. Other symptoms included loss of appetite (decreased appetite), chills, fever (pyrexia), fatigue, malaise, abdominal pain, diarrhea (diarrhoea), palpitations, heat intolerance (temperature intolerance) and 5lbs weight loss. She was treated by her primary care provider for suspected pharyngitis with a course of corticosteroids and antibiotics. Two weeks later, given worsening symptoms, was referred to the emergency room. On exam, she appeared anxious (anxiety) and was tachycardic (tachycardia) (124 beats per minute) and tachypneic (Tachypnoea) (28 breaths per minute). She had no lid lag, stare, thyromegaly or thyroid bruit. However, there was significant tenderness on palpation of the anterior neck (Tenderness). Laboratory evaluation was notable for Thyroid stimulating hormone (TSH)<0.01 uIU/ mL (0.39 - 4.0.8), free (Thyroxine) T4 5.19 ng/dL (0.58 - 1.64), free triiodothyronine (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 severe thyrotoxicosis (hyperthyroidism). Given Burch-Wartofsky score of 45, there was a concern for impending thyroid storm (thyrotoxic crisis). She received hydrocortisone, methimazole and beta blockers (corrective treatment) and was admitted to the intensive care unit. She responded dramatically to treatment and was discharged within 24 hours on prednisone and metoprolol (corrective treatment).  The 24-hour radioactive iodine uptake (RAIU) was 2.7% 2 weeks post admission, consistent with a diagnosis of subacute thyroiditis (thyroiditis subacute). Thyroid-stimulating immunoglobulin (TSI) was negative. Family history was negative for autoimmune disease. She continued on beta blockers with a steroid taper for 8 weeks. Thyroid function tests and inflammatory markers normalized within 3 months.   Author stated that: Thyroid storm is a rare, potentially fatal condition, affecting 1% of individuals with thyrotoxicosis. Although it can theoretically be seen in any cause of thyrotoxicosis, the most likely underlying etiology is Graves' disease. The rarity of thyroid storm due to thyroiditis makes the diagnosis challenging as the clinical and biochemical features overlap Grave's disease. Here we describe a rare case of severe thyrotoxicosis in a woman due to subacute thyroiditis that developed after receiving an influenza vaccine. Aside from the described patient, only three other cases of thyrotoxic crisis due to subacute thyroiditis have been reported in the literature. 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. Subacute thyroiditis is very uncommon after influenza vaccine, there have been 4 reported cases.  Seriousness criteria: medically significant, hospitalization for all events Action taken: not applicable Corrective treatment: hydrocortisone, methimazole and beta blockers; prednisone and metoprolol for all the events Outcome: Recovered for all the events Reporter's causality: Related; Sender's Comments: Sanofi company comment on dated 16-NOV-2023: This case involves a 30-year-old female patient who experienced subacute thyroiditis following severe thyrotoxicosis, thyroid storm/thyrotoxic crisis due to subacute thyroiditis and subacute thyroiditis following an influenza vaccine. Based on the limited information received the causal role of VACCINE cannot be excluded due to compatible temporal relationship of ADR to the VACCINE. Further information regarding concurrent condition during vaccination, previous vaccination, tolerance, allergic history, excluding alternative etiologies for the reported event are needed to fully assess this case. Based upon the reported information, the role of vaccine cannot be assessed completely.