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, see Surgery 1999 Dec;126(6):1167-71; Recommendations for management of cystic thyroid disease.

    For evaluation of a partly cystic nodule, the risk of cancer seems proportionate to the degree to which the nodule also contains solid components. In one study, if a partly cystic nodule was comprised of at least 50% solid tissue, the risk of malignancy approached 20%. On the other hand, largely cystic nodules with smaller solid components have a much lower risk of thyroid cancer, perhaps 5%. See Partially cystic thyroid nodules on ultrasound: probability of malignancy and sonographic differentiation Thyroid. 2009 Apr;19(4):341-6