Diagnosed with Hashimoto's after 2nd dose; This spontaneous case was reported by a consumer and describes the occurrence of AUTOIMMUNE THYROIDITIS (Diagnosed with Hashimoto's after 2nd dose) in a 63-year-old female patient who received mRNA-1273 (Moderna COVID-19 Vaccine) for COVID-19 vaccination.     Concurrent medical conditions included Hypothyroidism, Thyroid disorder NOS and Mitral valve disease (Mitral valve regurgitation). Concomitant products included LEVOTHYROXINE for Hypothyroidism.   In February 2021, the patient received first dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) 1 dosage form. In March 2021, received second dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) dosage was changed to 1 dosage form. In March 2021, the patient experienced AUTOIMMUNE THYROIDITIS (Diagnosed with Hashimoto's after 2nd dose) (seriousness criterion medically significant). At the time of the report, AUTOIMMUNE THYROIDITIS (Diagnosed with Hashimoto's after 2nd dose) outcome was unknown.      DIAGNOSTIC RESULTS (normal ranges are provided in parenthesis if available): On an unknown date, SARS-CoV-2 test: negative (Negative) Never had COVID-19 disease and her PCR test was always negative in the past.         No treatment information was provided. Company comment: This case concerns a 63-year-old, female patient with medical history of Hypothyroidism, Thyroid disorder and use of Levothyroxine, who experienced the unexpected events of autoimmune thyroiditis. The event occurred after the second dose of mRNA-1273.  As reported, the patient had history of Hypothyroidism prior to vaccination and she was taking Levothyroxine but the diagnosis of Hashimoto's came after the 2nd Moderna vaccine. The rechallenge was not applicable as events occurred after second dose and no further dosing was reported or planned, based on information provided. The above mentioned medical history remains a confounder. The patient already had hypothyroidism and was taking levothyroxine but was diagnosed after 2nd dose. Therefore, it is likely that Hashimoto's thyroiditis had onset prior to vaccination but the final diagnosis was made after 2nd dose, hence causality for the event is assessed as not related. The benefit-risk relationship of mRNA-1273 is not affected by this report.    This case was linked to MOD-2021-403160 (Patient Link).; Sender's Comments: This case concerns a 63-year-old, female patient with medical history of Hypothyroidism, Thyroid disorder and use of Levothyroxine, who experienced the unexpected events of autoimmune thyroiditis. The event occurred after the second dose of mRNA-1273.  As reported, the patient had history of Hypothyroidism prior to vaccination and she was taking Levothyroxine but the diagnosis of Hashimoto's came after the 2nd Moderna vaccine. The rechallenge was not applicable as events occurred after second dose and no further dosing was reported or planned, based on information provided. The above mentioned medical history remains a confounder. The patient already had hypothyroidism and was taking levothyroxine but was diagnosed after 2nd dose. Therefore, it is likely that Hashimoto's thyroiditis had onset prior to vaccination but the final diagnosis was made after 2nd dose, hence causality for the event is assessed as not related. The benefit-risk relationship of mRNA-1273 is not affected by this report.