thyroid came back just a little bit high; irregular heart beat; This is a spontaneous report received from contactable reporter(s) (Nurse). The reporter is the patient.  A 76-year-old female patient received BNT162b2 (BNT162B2), on 01Dec2021 as dose 3 (booster), single (Batch/Lot number: unknown) at the age of 76 years for covid-19 immunisation; levothyroxine, liothyronine (NP THYROID), (Batch/Lot number: unknown) for thyroid disorder. The patient's relevant medical history included: "Thyroid" (unspecified if ongoing). The patient's concomitant medications were not reported. Past drug history included: Levothyroxine for thyroid problem, reaction(s): "levothyroxine quit working". Vaccination history included: Bnt162b2 (Dose 1), for Covid-19 immunization; Bnt162b2 (Dose 2), for Covid-19 immunization.  The following information was reported: HEART RATE IRREGULAR (hospitalization, medically significant) with onset Dec2021, outcome "not recovered", described as "irregular heart beat"; THYROID HORMONES INCREASED (hospitalization) with onset Jan2022, outcome "recovered", described as "thyroid came back just a little bit high". The events "irregular heart beat" and "thyroid came back just a little bit high" required emergency room visit. The patient underwent the following laboratory tests and procedures: Blood thyroid stimulating hormone: fine, notes: Thyroid was retested and was fine; Heart rate: irregular. The action taken for levothyroxine, liothyronine was unknown.   Clinical course reported as: patient had the first booster 01Dec2021 after having the initial injections and about three weeks later she started getting an irregular heart beat. It was not the bivalent, the bivalent wasn't out yet. Product was the Regular Booster Pfizer first came out with. Everything was closed so she waited and went to the doctor the following week. Actually, got so bad she went to the Emergency Room to get it checked out. Saw that her thyroid came back just a little bit high. She consulted her thyroid doctor who told her to lower the dosage of her thyroid medication. Thyroid was retested and was fine but the irregular heart beat continued. Outcome: Slightly better, clarified at times it gets pretty bad. Not all the time but when she does have it, it is pretty irregular and makes her tired. Causes skipped beats and premature contractions. Slows her down and she has to rest more. Thyroid came back a little high: Went to the Emergency Room before she went to the doctor so right after the first of the year, first week of the following year. Had to wait so she was not sure when it resolved but she had to wait 6 weeks or 2 months to be retested. Thyroid medicine works that way to stay in the system a long time to regulate or re-regulate. Went to her thyroid doctor and was told it had resolved and the numbers were fine but she was still having an irregular heart beat. She clarified she has had a thyroid problem since her 20s. Thyroid level goes up and down, not unusual. She may be just not as tired depending on if it is low or high. Confirmed her thyroid problem was before receiving the Regular Booster. Happened before the Regular Booster and it goes up and down. Sometimes she went for years and has no change in dosing. Thyroid was a difficult organ. Had received her first two doses of the Pfizer Vaccine at intervals and this interval for the Regular Booster was a little longer than the usual interval between. In her mid 20s, 52 year ago about. Actually she was taking levothyroxine and then the levothyroxine quit working so she switched over to Armour Thyroid then went to NP Thyroid because Medicare does not cover the name brand at all for levothyroxine weather it works on it or not.    The information on the batch/lot number for BNT162b2 has been requested and will be submitted if and when received.; Sender's Comments: Based on the available information in the case, an association between the reported events and bnt162b2 cannot be ruled out.  The impact of this report on the benefit/risk profile of the Pfizer product is evaluated as part of Pfizer procedures for safety evaluation, including the review and analysis of aggregate data for adverse events. Any safety concern identified as part of this review, as well as any appropriate action in response, will be promptly notified to regulatory authorities, Ethics Committees, and Investigators, as appropriate.