Thyroid cysts are actually very uncommon lesions of the thyroid gland. Unfortunately, patients and doctors alike sort of casually throw the word around, when, in fact, the abnormality is actually something a bit more complex. I like to describe a true cyst as being just like a water balloon (without the tied knot on the outside). That is, it is a sac filled with liquid, and only liquid. These are virtually always benign. I have seen four, or perhaps five, small cancers in the wall of a true cyst throughout my 32 year career in thyroid surgery, but this is more a reflection of the vast number of thyroid tumors we take out than anything else.

Usually the word “cyst” pops up as a result of an ultrasound examination. The ultrasound report says something like “there is a cystic lesion found...etc.” In most cases what is called a cyst is really a tumor of the gland, a portion of which has undergone a process called “cystic degeneration” or liquification. It’s also possible that some bleeding occurred within the tumor and the “cystic” area within the tumor is actually blood that will probably be reabsorbed by the body in time. A more accurate description of these types of thyroid tumors, or nodules, is that they are “complex”. A complex lesion has both solid and cystic components, and they are actually tumors, not cysts.

Usually tumors of this type are benign and frequently the diagnosis turns out to be “follicular adenoma with cystic degeneration and evidence of recent hemorrhage”. Cancer in these tumors is still very possible, unlike what we see in true thyroid cysts, and this is why it is so very important to accurately describe what the abnormality is. A cyst is a cyst, a tumor that is partially cystic is still a tumor.