Primary hydatid cyst of the thyroid glands: two case reports and a review of the literature

Echinococcosis, a parasitic infection affecting humans, causes around 871,000 disability-adjusted life-years (DALYs) globally, each year [1]. Ninety-five percent of all human echinococcal diseases are caused by hydatid cyst disease (Cystic Echinococcosis) [2]. Controlling the disease costs three billion dollars annually worldwide [1].

Hydatid cyst is endemic in areas with a mild climate such as the Middle East, Southeast Asia, Mediterranean countries, and South America [3]. In Iran, the prevalence of human hydatid cyst was 4.2%, with most cases being female and living in rural regions [4].

The parasite life cycle consists of three different stages: eggs (in the environment), adult worms (in the definite host intestine), and metacestodes (in the intermediate host) [5, 6]. In the intermediate host, an oncosphere larva is released into the intestine following the ingestion of an egg. The larva then penetrates the lamina propria and gets into the liver or other organs through the blood or lymphatic system. In the internal organs, the larva forms hydatid cysts (metacestode larvae) with an inner germinal layer and an outer laminated layer containing protoscolices [7, 8].

Hydatid cyst occurs during the larval stage, and humans are the accidental intermediate hosts. The liver (65%) and the lungs (25%) are the two most frequently affected organs in humans [9]. The hydatid cyst rarely attacks the thyroid gland, even in endemic countries [2, 9‐11]. The larvae can bypass the liver and lungs and settle in the thyroid gland via systemic circulation [12, 13]. The thyroid hydatid cysts usually accompany the hepatic and pulmonary cysts, and the isolated primary form is scarce, which may mimic malignancies. The growing cysts can compress the adjunct structures, eventhough most cases are asymptomatic. Moreover, anaphylactic shock and death may occur following cyst rupture [9].

All general practitioners, radiologists, surgeons, and pathologists should be aware of the presenting signs and symptoms and the paraclinical features of primary hydatid Cysts of the thyroid gland. Therefore, this study aims to present two female patients with primary hydatid cysts of the thyroid gland and highlight the clinical and paraclinical features. The Institutional Review Board and Research Ethics Committee of Shiraz University of Medical Sciences approved the study with the ethics approval number: IR.SUMS.REC.1401.185.

The hydatid cysts frequently affect the liver and lungs. However, other organs' involvement, such as the pancreas [14], trapezius muscle [15], axillary region [16], breast [11], and ovaries [17], have been addressed. Even in highly endemic countries, including Iran, the hydatid cysts of the thyroid gland are rare [9].

The pathogenesis pathways are somewhat clear. The oncosphere larva enters the systemic circulation after bypassing or passing through the hepatic microcirculation to reach the thyroid gland (primary or secondary hydatid cysts, respectively). Although the gland is a well-nourished organ with a high blood flow, the hydatid cyst is rare due to the small diameter of its arteries [18, 19].

Reddy and M. Thangavelu reported the first case of thyroid involvement in 1946 [20]. Since then, several cases have been reported worldwide [2, 9, 10, 13, 21‐30]. Iran is an endemic country, and some studies have reported thyroidal involvement. Geramizadeh reviewed all cases reported between 1990 to 2011; only four cases of the thyroid gland involvement have been reported. All of these cases were females and presented with compressive symptoms [11]. To the best of our knowledge, only 14 case studies (17 patients) have discussed the thyroid hydatid cyst since 2012, as detailed in Table 1. The primary hydatid cysts of the thyroid are usually asymptomatic. However, they may mimic thyroid malignancies as their size increases, and the cyst attaches to adjacent structures such as the esophagus, trachea, and recurrent laryngeal nerve, leading to compressive symptoms [31, 32]. Unlike our patients, seven studies have reported these symptoms, such as hoarseness and dyspnea, since 2012 [2, 9, 21, 23, 25, 27, 29].

Our patients did not report any significant changes in the size of the masses. Generally, the hydatid cysts are not known as rapidly growing neck masses, although in some cases, they may progressively grow [24‐27]. Moreover, the primary hydatid cysts are usually solitary cystic lesions affecting a single thyroid lobe; however, more than one cyst can also be detected. In the case report by Aydin et al., two cysts in the two thyroid lobes were found [24]. Similar to most previous studies, a solitary cyst in the right thyroid lobe was detected in our patients.

The diagnosis is based on clinical and paraclinical analyses such as ultrasound and FNA findings. However, most cases are diagnosed after surgery, and post-surgical pathology remains the gold standard of diagnosis [33, 34]. The literature showed that almost all cases were diagnosed through post-surgical histopathology findings. However, the patient management in two studies reported in the USA [26] and India [10] was solely based on FNA since 2012 (Table 1). In both cases, the scolices/protoscolices hooklets were seen in FNA. We detected the same findings in the FNA of our two patients; however, we decided to examine the histopathological features of the removed gland for further confirmation and to rule out any concomitant malignancies.

When a hydatid cyst is expected, using FNA is controversial regarding the likelihood of cyst rupture. Various complications from mild allergic to anaphylaxis reactions have been reported following the FNA of the hydatic cysts [35‐37]. The most common complications are anaphylaxis following cyst rupture and abscess formation [2, 12]. Neither of our cases reported in this study showed complications related to cyst rupture. Of course, in both cases, after FNA, the liquid contents inside the cyst were completely drained. Moreover, the surgical field was irrigated with hypertonic saline during the operation as the previous studies have recommended using hypertonic saline to prevent the occurrence of intra-operative anaphylaxis [38].

In our study, patients presented with neck lumps without any other symptoms. The US was the first modality for evaluating the neck mass, which showed cystic lesions with intraluminal solid parts in both cases. The US helps distinguish solid or cystic lesions of the neck, and if a detachment of the membrane of the cyst with a multilocular appearance exists, it can suggest a hydatid cyst [39]. Unfortunately, in our cases, the ultrasound findings did not indicate the hydatid cysts. Since the US is an operator-dependent diagnostic tool, high clinical suspicion may help the radiologist detect hydatid cysts.

Surgical resection is the treatment of choice, and antiparasitic medications like Mebendazole or Albendazole may be administered when there is a contraindication for surgery [40, 41]. According to the current literature, only three cases have been managed using antiparasitic medications. A 30-year-old female reported by Dey et al. received only Albendazole treatment (400 mg/day) for 28 days without any subsequent surgery [10]. One of the cases reported by Yilmaz et al. received mixed treatment. She was an 18-year-old female who received Albendazole, followed by left lobectomy and isthmectomy [13]. The last patient was a 48-year-old female with bilateral pulmonary and thyroid cysts. She received Albendazole for six weeks but did not improve. The thoracotomy and bilateral lobectomy removed the complicated cysts [30]. We did not use antiparasitic drugs, and surgical removal was done for both cases. Of note, they have no signs of recurrence or other remote organ involvement at the 6-month follow-up.

Considering the endemicity of hydatid cysts in Iran [4] and the Middle East [13, 24, 29, 35] and the occurrence of thyroid involvement, physicians and stakeholders must know about the common epidemiological, clinical, and paraclinical features. Moreover, it is essential to consider hydatid cysts in endemic areas as one of the differential diagnoses for cystic lesions in the thyroid. In conclusion, we reported two females with primary thyroid hydatid cysts. They had no local or systemic symptoms. They were treated through thyroidectomy, and no recurrence was detected at the 6-month follow-up. The current study emphasizes that high clinical suspicion is needed to differentiate thyroid hydatid cysts from other cystic lesions in regions.