The exceedingly rich vascularity of endocrine organs predisposes to an increased likelihood of developing metastatic deposits.¹,⁴,⁵ The thyroid gland is affected by widely disseminated disease.⁶ Curiously, the abnormal thyroid gland (affected by some other disease) tends to be more frequently affected than a normal gland, perhaps suggesting alterations in vascularity or blood flow may contribute to metastatic disease developing.⁵,⁷

Identification of metastatic disease in surgical pathology material is seen in up to 7.5% of thyroid glands,⁷,⁸,⁹ although up to 25% of autopsied patients with disseminated malignancies will have thyroid gland metastatic deposits.¹,¹⁰,¹¹ The apparent increase in cases recently may be related to advancement in radiographic studies, improved treatments resulting in prolonged survival, and increased frequency of fine needle aspiration (FNA) for thyroid gland nodules.⁸,¹²,¹³ Patients of all ages are affected, although elderly patients are more frequently noted to have disease (mean age of 63.8 years for 340 reported cases) (Table 18.1).⁵,⁷,⁹,¹⁰,¹²,¹⁴,¹⁵ There is a slight gender predilection (female > male, 1.1:1), but the gender differences are expected with breast and gynecologic primaries compared to prostate primaries.⁴,⁵,⁷,⁸,¹⁰,¹⁵,¹⁶,¹⁷,¹⁸ The clinical presentation is usually a mass within the thyroid gland, although it may be masked by underlying thyroid gland disease.¹,⁵,⁸,¹⁰,¹²,¹³ Occasionally, a rapidly enlarging thyroid mass is seen.¹⁹ Hoarseness (compression of the recurrent laryngeal nerve), dysphagia, dysphonia, neck pain, and even hemoptysis may also be noted.⁵,¹⁴,¹⁵,¹⁹ In rare circumstances, hyperthyroidism may be the presenting symptom, apparently because of thyroid parenchymal destruction and hormone release.¹⁵ The thyroid gland metastatic deposit is the initial presentation of an occult primary tumor in up to 40% of patients,⁸,¹² although this seems to be most frequently noted in kidney primaries.⁵,²⁰ This finding may help to direct the search for an unknown primary. Radiographic studies may help, with ultrasonographic studies exhibiting unilateral or bilateral, multiple, ill-defined, infiltrating, hypoechoic nodules with inhomogeneous texture. There are no microcalcifications, although necrosis and hemorrhage can be seen.¹²,¹³,¹⁵,¹⁶ Although nonspecific, bilateral, multiple nodules without microcalcifications may suggest metastatic disease in patients with a known nonthyroidal primary tumor (Fig. 18.1). FNA or core needle biopsy (often ultrasound guided) may help to confirm this impression and is the initial study of choice in this setting.⁴,⁵,⁸,¹⁰,¹¹,¹²,¹³,¹⁴,¹⁵,¹⁷

The time to the appearance of metastatic disease from the identification of the primary tumor ranges up to 22 years, a finding suggesting an exceedingly careful clinical history is crucial to making an accurate diagnosis.⁴,⁵,⁶,⁹,¹¹,¹²,¹⁵ However, approximately 80% of metastases develop within 3 years of the primary tumor resection, with the exception of renal cell carcinoma that is notorious for having a long latency period.⁴,⁵,⁷,⁹,¹⁰,¹²,¹⁴,¹⁵

In clinical surgical pathology series, metastatic deposits are identified at a higher frequency in abnormal glands, such as, adenomatoid nodules, thyroiditis, and follicular-pattern neoplasms.⁵,¹⁰,²¹ Interestingly, the metastatic deposits may be found within or adjacent to primary thyroid tumors (Figs. 18.2B and 18.3).⁵,²¹,²² There is variability depending on whether the reported series is based on clinical or autopsy material. Carcinomas are most common (approximately 80%), with the most common primary sites in order of frequency being kidney, lung, breast, and gastrointestinal tract (esophagus, stomach, and colon); leiomyosarcoma (usually uterine, but gastrointestinal tract primaries are reported) and skin melanoma are the most common sarcomas (Table 18.1).¹,⁴,⁵,⁶,⁷,⁸,⁹,¹⁰,¹¹,¹²,¹⁴,¹⁵,¹⁶,¹⁷,²³ Isolated case reports from virtually every other organ have been reported, such as salivary gland, pancreas, bladder, tongue, nasopharynx, prostate, ovary, parathyroid, testes, and bone, to name just a few.⁸,¹⁰,²⁴,²⁵