The presence of abnormal histological elements within the intact thyroid gland has only been recorded in very few cases. In addition to distant metastases to the thyroid, a number of ectopic tissue rests and unusual cysts have been described within the thyroid gland [4, 5]. In this article, we explain the existence of a true intrathyroidal lymph node which is the second case of papillary thyroid cancer with intrathyroidal lymph node metastasis. The first case was reported by Shawky [6] in 2017, a 63-year-old female in New Zealand with suspicious for Hurthle cell neoplasm versus oncocytic variant of papillary thyroid cancer. PTC metastases were indicated by papillae and nests of atypical polyhedral cells with vesicular nuclei, inclusions, and grooves intranodally.
The thyroid gland is made up of follicles, which produce colloid and are surrounded by follicular cells, calcitonin-producing parafollicular cells, also known as C-cells, a delicate connective tissue stroma, and vascular elements [7]. Any other histological elements found within the thyroid gland are abnormal. Non-metastatic and metastatic are two types of abnormal intrathyroidal tissue. Ectopic rests of non-thyroid tissue or irregular cysts are both thought to be the product of dysembryogenesis in non-metastatic intrathyroidal tissue. Intrathyroidal metastasis, on the other hand, often means spread from a distant primary malignancy [8].
While the presence of LN tissue within the parotid gland is a common occurrence [9], a similar finding within the thyroid is extremely uncommon, having only been identified once in a 40-year-old Egyptian woman with multinodular goitre [10]. To the best of our knowledge, this is the second case of a metastatic intrathyroidal LN that has been identified. The finding is not lymphocytic infiltration of the thyroid parenchyma; rather, they are a true LN with germinal centres and an intact capsule, lying entirely inside thyroid tissue and displaying features of papillary thyroid carcinoma metastasis. A divergence from the standard developmental phase may be one reason for such findings [11]. During the 8–9th weeks of pregnancy, there may be abnormal extension of the Juglo-axillary lymph sacs or abnormal sprouting of the lymphatic primordials in the cervical area. Since the presence of an intrathyroidal LN may cause the FNAC to contain lymphocytes and be misinterpreted as Hashimoto’s disease or lymphoma, routine sonographic guidance for all FNACs, including well palpable nodules, is important [12]. The presence of a metastatic intrathyroidal LN, with small differentiated thyroid cancer as in our case, should be regarded as N1a if the neck is node negative. However, according to the updated American Thyroid Association 2009 recommendations, this will otherwise up-stage the disease to stage III in patients older than 45 years, potentially affecting the prognosis and management plan [13].
Thyroid metastasis appears to be uncommon, with a prevalence of 5.5% among patients who had their thyroids removed for cancer [14]. Between 2000 and 2010, thyroid metastases from renal cell carcinoma accounted for 48.1 per cent of non-thyroid malignancies (NTMs) metastases to the thyroid gland, with a proclivity to spread into the jugular veins [15]. Renal cell carcinoma, breast cancer, and lung cancer are the most common primary tumour sites; however, there is no complete agreement on which cancers most frequently metastasize to the thyroid, as this depends on many factors such as the epidemiology and clinical behaviour of the primary cancer, as well as the diagnostic methods used [16]. Thyroid metastasis can pose both diagnostic and therapeutic challenges. It is possible that the metastatic focus is very limited and clinically undetectable. In 50 to 60% of patients with no palpable nodules, incidental thyroid nodules are discovered at post-mortem [17]. Furthermore, it has been reported that the delay between the diagnosis of the primary cancer and the thyroid secondary has been as long as 26 years [18]. The presence of metastatic lesions in the thyroid gland is a bad omen in terms of prognosis, with patients dying shortly after the diagnosis of thyroid gland metastasis is made [14]. As a result, any thyroid nodule in a patient with a history of cancer should be treated with caution and extensively examined using FNA, preferably in combination with immunohistochemistry and sonography [12].
