In adults, the most common cause of sudden death is coronary heart disease or defects in the cardiac conduction system; however, there are many cases of sudden death occurring from other causes such as fatal pulmonary thromboembolism. Several risk factors are recognized, including hospitalization, surgery, obesity, pregnancy, the use of oral contraceptives, traumatic fractures, and genetic conditions, which cause hypercoagulable states such as factor V Leiden mutations. Although many risk factors have been identified, the mortality rate is still high. An association between thyroid disease and coagulation disorders has previously been demonstrated. However, scientific literature does not yet agree on the association between thyroid diseases and pulmonary thromboembolism. Excessive thyroid hormones can determine an alteration in the hemostatic system through different mechanisms, which lead to a hypercoagulable state. In forensic literature, there are no reports of fatal thromboembolism in patients with thyroid disorders. We reported a case of a 41-year-old woman who died during sexual activity. The autopsy findings highlighted the presence of massive pulmonary embolism. Histopathological analysis showed structural anomalies of the thyroid gland. A retrospective analysis of her medical history and possible genetic alterations were performed.