Thyroid cancer is a malignant tumour that starts in the cells of the thyroid. Malignant means that it can invade, or grow into, and destroy nearby tissue. It can also spread, or metastasize, to other parts of the body.
The thyroid is part of the endocrine system. It is a small gland in the front of the neck below the larynx (voice box) and near the trachea (windpipe). It has a right and left lobe, one on each side of the trachea. The lobes are joined by a thin piece of tissue called the isthmus.
The thyroid is mainly made up of follicular cells and C cells. Follicular cells make thyroid hormones. These hormones help break down food into energy. They also help control body functions such as body temperature, heart rate and breathing. C cells (also called parafollicular cells) make the hormone calcitonin, which helps control the level of calcium in the blood.
Cells in the thyroid sometimes change and no longer grow or behave normally. These changes may lead to non-cancerous, or benign, conditions such as hypothyroidism, hyperthyroidism, thyroid nodules, thyroiditis or goitre.
In some cases, changes to thyroid cells can cause cancer. The most common types of thyroid cancer are papillary carcinoma and follicular carcinoma. They are usually grouped together as differentiated thyroid cancer, which makes up more than 90% of all thyroid cancers.
Less common and rare types of thyroid cancer can also develop. These include poorly differentiated carcinoma, anaplastic carcinoma, medullary carcinoma, non-Hodgkin lymphoma and soft tissue sarcoma.
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