Fine-needle aspiration is essential in order to obtain a sample of the thyroid tissue to allow for microscopic examination. This allows an experienced pathologist to differentiate<unk>  from other diseases, such as other forms of thyroid cancer. It is very important to distinguish between<unk>  and poorly-differentiated thyroid cancer and this distinction can be difficult to make. The presence of PAX-8 positive staining and association with a different thyroid cancer that is adjacent to the<unk>  support the diagnosis.<unk>  is divided into several different subclasses based on its microscopic characteristics. These include sarcomatoid, squamoid, osteoclastic, paucicellular, rhabdoid, and carcinomasarcoid variants. As of 2019, despite the fact that these<unk>  subtypes are recognized, this classification has not led to differences in management.<unk>  is always considered to be stage IV when it is diagnosed. There are no reliable laboratory tests for<unk> . Ultrasound imaging of<unk>  lesions reveals a hypoechoic mass (appears dark on ultrasound) with invasion of the local structures and may help to better characterize the presence or absence of neck lymph node metastases. If surgery is planned, however, then a contrast-enhanced computed tomography (CT) scan of the neck must be performed. A PET scan is preferred for staging<unk>  but a CT scan of the neck, chest, abdomen, and pelvis can be substituted if the former is unavailable. Magnetic resonance imaging (MRI) of the brain is also recommended to assess for distant metastases. Differential diagnosis In addition to<unk> , a rapidly enlarging neck mass prompts consideration of several other important diagnoses. These include other cancers such as primary thyroid lymphoma, poorly differentiated thyroid cancer, sarcomas, and metastases from cancers of the upper digestive tract and respiratory tract. Squamous cell carcinoma of the thyroid gland is a rare cause of this presentation.
Anaplastic thyroid cancer
Thyroid lymphoma
Follicular thyroid cancer
Papillary thyroid cancer
Medullary thyroid cancer
Squamous-cell thyroid carcinoma
Poorly differentiated thyroid cancer
Thyroid adenoma