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Thyroid cysts represent enlarged fluid-filled regions of the thyroid that may be small (less than 1 cm) or quite large and sometimes arise very suddenly. A cyst, by definition, contains fluid. Thyroid nodules may be entirely cystic, in which case there are no solid components detectable within the fluid. Alternatively, the nodule may be complex, and contain both fluid and solid components. Cystic nodules may expand and enlarge suddenly sometimes due to hemorrhage or bleeding within a smaller pre-existing nodule. In some cases, rapidly enlarging cysts may produce symptoms in the neck, including pain, trouble swallowing, and rarely, compression of vocal cords leading to a change in voice quality.

Cysts that are entirely fluid filled have a much lower risk of harboring a small thyroid cancer compared to cysts that have solid components. Diagnosis of a cystic thyroid lesion may be made at the time of ultrasound, or following a thyroid aspiration biopsy, when fluid is obtained from the thyroid lesion. Complete spontaneous resolution of a thyroid cyst may occur, but is uncommon (~ 15 % of cases). Most large cysts or complex cysts should be aspirated with a fine needle to rule out the possibility of malignancy. Ultrasound-guided biopsies of complex cystic nodules may be particularly useful for ensuring that the biopsy material contains thyroid cells, and not just cystic fluid. For an overview of the diagnosis and management of cystic thyroid disease.
Source(s):
http://www.mythyroid.com/thyroidcysts.ht…