DISCUSSION
The first reported case of thyroid involvement with hydatid cyst was in 1946 (4). Since then, there have been reports from the endemic regions, however to our knowledge, none have been reported from South Asia.
Hydatid cyst of thyroid can develop either primarily (affecting only the thyroid) or secondarily (involving multiple organs). Possible pathogenesis is emergence of oncosphere larvae in gastrointestinal tract followed by passage to systemic circulation either after bypassing the liver or passing through liver  and  reaching to thyroid gland (5). Humans are accidental hosts and contract the disease by accidentally ingesting contaminated food and water or through contact with a definitive host (10). In our case, the patient had a history of exposure to cattle and sheep similar to that in previous cases.
Symptoms of hydatid cyst in thyroid depends upon  size and adherence of the lesion to surrounding structures with most of the patients are asymptomatic except for non-progressive swelling as ours The  slow growing pattern explains clinical dormancy for several years(11). However, patients may complaint of difficulty breathing, swallowing,  hoarseness , anaphylaxis, pyogenic abscess formation and cystotracheal fistula secondary to erosion of tracheal wall by cyst wall (4).
USG is a routine investigation to visualize cyst and daughter vesicles. CT /MRI are used to assess the location of cyst in relation to its surrounding structures (4).
Despite having limited sensitivity and specificity in primary thyroid hydatid cyst disease, immunological methods like indirect hemagglutination, latex agglutination, ELISA, and immune electrophoresis  are important modality if diagnosis remains unclear (12).
The role of FNAC in preoperative diagnosis of thyroidal hydatid cyst is controversial despite higher sensitivity and specificity pertaining to the high risk of anaphylactic reactions and the dissemination of cystic elements leading to inflammation making surgery difficult (4).
However, our patient didn’t have immediate or late reactions following the USG guided FNAC.
Ultrasound scan of the neck was performed in our patient which was non-conclusive and FNAC findings revealed diagnosis of hydatid cyst of thyroid to detect the unilateral hydatid cyst on the left side of thyroid gland.  To the best of our knowledge, there has been only a case of thyroid hydatid cyst diagnosed by FNAC before the surgery as in our case (6).
Although post-surgical histopathology remains gold standard, an integrative approach including patient history, place of origin, examination, imaging examinations like USG, CT scan, magnetic resonance imaging (MRI), scintigraphy, serological examination and FNAC (4).
Surgical resection is the preferred treatment option. Administering anti-parasitic drugs like Mebendazole or Albendazole may be alternative treatment when there is a contraindication for surgery and as postsurgical adjuvant chemotherapy for preventing recurrence. Follow-up is recommended after 3 months, 6 months and after a year of surgical operation (5)(7). The patient’s condition significantly improved following her surgical operation, and no antiparasitic medications were needed as there were no complications after surgery.