An update on the current epidemiological status of metastatic neoplasms to the thyroid.

Secondary tumours to the thyroid gland are uncommon with an overall prevalence of 5.9% in autopsy studies. In recent clinical series, secondary thyroid cancer is seen in only 1.9% of malignant thyroids. There is no gender predominance both overall (female to male 1.07:1) and when stratified by common histological subtypes (renal cell carcinoma, lung adenocarcinoma and colorectal adenocarcinoma). The median age of patients diagnosed with metastatic thyroid tumours in major studies ranges from 54 to 68 years. Metastases are more frequent in patients with pre-existing or concurrent thyroid pathology. In autopsy studies, the most common primary sites are breast carcinoma and lung carcinoma. Renal cell carcinoma, lung carcinoma and breast carcinoma predominate in clinical series. Upper aerodigestive tract primaries often directly infiltrate the thyroid gland. The underlying frequency of a histological subtype, geographic prevalence and aggressiveness of primary cancer likely contributes to the incidence of metastasis in the thyroid gland. This is seen in case series from Asia where gastric and oesophageal primary cancers predominate. Secondary thyroid cancer can present metachronously (60%), synchronously (34%), or as the first presentation of the underlying cancer (6%). Late metastases and first clinical presentations of disease often originate from renal cell carcinomas while synchronous cases tend to originate from the lungs. Other common primary sites for first presentation of secondary thyroid cancer include the lung and oesophagus. Although rare, secondary thyroid cancer should be considered as a differential particularly in patients with previous malignancy, such as from the kidney, lung, or breast.