Malignant melanoma presenting as a thyroid mass with synchronous retroperitoneal metastases in a 55-year-old male: a case report.

Background: Malignant melanoma (MM) is an aggressive tumor characterized by a high tendency for early metastasis. Although most cases have identifiable primary sites, melanoma of unknown primary (MUP) presents as metastatic disease without detectable primary lesions. MM presenting as a thyroid mass with concurrent metastasis to the spleen-kidney interspace is extremely rare, and the occult nature of the primary lesion also poses a significant diagnostic challenge, often leading to confusion with aggressive primary thyroid malignancies. Case Description: A 55-year-old male with an unremarkable medical history presented with a 3-month history of a rapidly enlarging, painless left neck mass. Physical examination revealed a firm 5-cm mass in the left thyroid lobe and a similarly sized palpable mass in the right supraclavicular region. No suspicious lesions were noted on the skin or mucous membranes. Conclusions: Advanced melanoma should be included in the differential diagnosis of rapidly expanding, atypical thyroid masses, even in the complete absence of a known primary lesion. Clinicians must utilize fine-needle aspiration cytology and a targeted immunohistochemical panel to differentiate metastatic melanoma from primary anaplastic or medullary thyroid carcinomas early. Whole-body