The underlying cause of a growth in thyroid tissue, the source of thyroid nodules, of which a thyroid cyst is a variety, is unknown, explains Cleveland Clinic. However, degenerating thyroid adenomas are the most common cause of thyroid cysts specifically, according to Mayo Clinic. A thyroid adenoma is an overgrowth of normal thyroid tissue.
A thyroid cyst is a growth within the thyroid gland that is either completely fluid-filled or partially solid and partially fluid-filled, explains Cleveland Clinic. Individuals with an iodine deficiency or a family history of nodules are at higher risk of developing a thyroid nodule. The risk for developing a thyroid nodule also increases with age. Moreover, females more frequently develop nodules than men. Although thyroid nodules are almost always benign, less than 5 percent of nodules are cancerous.
The types of thyroid nodules other than thyroid cysts are colloid nodules, inflammatory nodules, hyperfunctioning thyroid nodules, multinodular goiters and thyroid cancer, according to Cleveland Clinic. A colloid nodule is a large, benign overgrowth of normal thyroid tissue. Inflammatory nodules result from the chronic inflammation of the thyroid gland. A multinodular goiter is an enlarged thyroid composed of multiple other nodules. Hyperfunctioning thyroid nodules produce thyroid hormone, which can cause hyperthyroidism, a condition that sometimes causes sudden cardiac arrest, hypertension or arrhythmia.