Introduction and objective:

Metastatic tumors in thyroid complicate the diagnosis, treatment, and prognosis of the patient. The objective is to determine the prevalence of metastasis in the thyroid gland reported in the medical literature and to examine the primary tumors that most frequently metastasize to the thyroid gland.

Method:

A systematic bibliographic review was undertaken in the PubMed, The Cochrane Library and Scopus databases. The selected articles were divided into two groups, clinical series of patients in whom thyroid metastasis were found (group A) and series of findings of metastasis in thyroid at autopsy (group B). Prevalence meta- analysis were performed for each group of articles following the random effects model.

Results:

The prevalence in each group with its 95 % confidence index was 0.00479 (0.002-0.007) for group A and 0.0362 (0.014-0.059) for group B. The prevalence of metastasis found at autopsies was 6.67 times higher than in clinical studies. In group A the mean age was 60.82 and in group B it was 57.20. In both groups, metastases found in the thyroid were more frequent in the female sex. The location of the primary tumor was different in both groups, in group A it was kidney cancer and in group B it was breast cancer. The variability of the prevalence of thyroid metastasis in the different articles of both groups makes this study highly heterogeneous (index I2 and Q). The funnel plots of both groups indicated high publication bias.

Discussion:

The different prevalence between clinical series and autopsies may imply that the detection of thy- roid metastases in the clinic is underdiagnosed. The reason for this could be that intrathyroid metastases appear asymptomatically, being diagnosed as an incidental finding at autopsy. On other occasions they present as a thyroid nodule years after the primary tumor, which determines the diagnosis.

Conclusions:

The prevalence of thyroid metastases is higher in autopsy series than in clinical series (up to 7.58 times more frequent in our study). Intrathyroid metastasis are probably underdiagnosed because they are not clinical, and they are diagnosed as a casual finding at autopsy. The most frequent primary tumors were the kidney (clinical series) and the breast (autopsy series).

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