Metastatic thyroid tumors are an uncommon entity, accounting for approximately 2 - 7.5% of thyroid malignancies. The most common primary sites are kidney, gastrointestinal tract, lung, head and neck, and breast. The presence of smooth muscle cell malignancies in the gland, both primary and metastatic, is extremely rare, with only a few metastatic leiomyosarcoma cases having been reported in the literature.

We report a case of a 72-year-old female patient with a history of uterine leiomyosarcoma, presenting with a right thyroid lobe nodule 12 years after the primary tumor excision. Fine needle aspiration cytology (FNAC) was classified as Bethesda V and a total thyroidectomy was performed, leading to the identification of the neoplasm. Additionally, systematic research of the literature was conducted in order to summarize and compare the reported cases of metastatic leiomyosarcomas of the thyroid, regarding their clinical features, management and outcomes.

In total, 24 cases of thyroidal metastatic leiomyosarcomas have been published. The most common primary site was uterus and the interval between the first diagnosis and the metastasis reached up to 10 years. Preoperative diagnosis with FNAC was achieved only in 4 cases and surgery was performed in the majority of the lesions.

Metastatic leiomyosarcoma of the thyroid is a rare and often misleading finding, posing challenges in differential diagnosis, due to its nonspecific clinical and radiologic features and inconsistent cytologic credibility. It is usually an indicator of disseminated disease and poor prognosis, although cases of 5-year survival have also been reported.