The small bowel obstruction was treated conservatively and resolved uneventfully. A computerised tomography (CT) scan performed as part of the acute investigation incidentally revealed a right upper lobe lung opacity which was avid on subsequent positron emission tomography (PET) scanning (Figure 1). On CT guided biopsy this was shown to be an adenocarcinoma contacting the pleura, histopathologically consistent with metastatic deposit from a colonic primary, with further tiny right lung nodules of minimal PET avidity also seen.

PET scan also revealed a left thyroid nodule (Figure 2). The patient was euthyroid throughout. Following multidisciplinary team (MDT) discussion an ultrasound (USS) and fine needle aspiration biopsy (FNAB) were performed on the thyroid nodule. Thyroid FNA was suspicious for papillary cell carcinoma with THY5 on immunohistochemistry.

Studies from the US and UK [5, 7] place kidney, followed by colorectal, lung, and breast as the most common primary sources for thyroid metastases, while an Asian series indicate malignancies of the gastrointestinal tract, especially of the upper GI tract, are also responsible [7]. Other reported primary tumours include neuroendocrine tumours, sarcoma, and more rarely melanoma [2, 3].

The timing of diagnosis of metastases to the thyroid is variable from time of initial diagnosis to years after treatment. The majority of patients are euthyroid at presentation, with thyrotoxicosis rarely seen [7]. When it does occur it is thought to be due to hormones leaking from the gland following neoplastic damage [3]. Metastases to the thyroid can present as a single nodule or as multiple foci within the gland [4]. 72% of patients with metastases to the thyroid in one study presented with a clinically detectable thyroid nodule [7], with other studies reporting signs of extrinsic compression at presentation [2, 3, 5]. In contrast to the much greater preponderance of primary thyroid tumours in women than men, the picture regarding metastases of nonthyroid malignancies to the thyroid gland is less clear. Some studies have reported a slightly higher incidence in women than men [5], but other series report no sex bias [7]. A recent review of the last 10 years suggests a female to male ratio of 1.4:1 [3].

Investigation of TM usually proceeds as with the assessment of any thyroid nodule-history, clinical examination, followed by USS and FNAB, with CT and PET imaging also playing a role [8]. Metastases to the thyroid typically appear ultrasonographically as hypoechoic masses with poorly defined margins and increased vascularisation. Calcification is rarely seen, in contrast to many primary thyroid tumours [7]. Immunohistochemistry is key in the diagnosis of metastases to the thyroid and the identification of the primary site. A nonthyroidal primary site may be indicated by a failure to react to thyroglobulin, TTF-1, and calcitonin [7], and indeed metastatic thyroid tumours are almost universal in their failure to react to thyroglobulin [6]. It is important to note however that the majority of anaplastic thyroid carcinomas also do not have a positive reaction to thyroglobulin [7].