The thyroid is a butterfly shaped gland located just under the skin at the base of the neck that regulates the body's energy rate, including heart pulse and blood pressure. The thyroid can become enlarged for many reasons, including production of too much or too little thyroid hormone, reports MayoClinic.com. Enlargement on just one side, however, is usually suggestive of a few conditions, including a nodule or cancer.
A thyroid cyst is filled with fluid and ranges in size from less than one third of an inch to over an inch, according to MayoClinic.com. Most are benign, although complex cysts, which have solid parts too, are more likely to be cancerous.
Solitary Colloid Nodule
A single colloid nodule, which is overgrowth of normal tissue, can appear on one side of the neck, reports MayoClinic.com. Colloid nodules are benign and do not spread outside of the thyroid gland.
Rarely, an inflammatory nodule may develop on one side of the neck. It can be caused by chronic swelling of the thyroid gland or be set off by pregnancy, which is called postpartum thyroiditis, reports MayoClinic.com.
Thyroid cancer, which is rare, usually appears as a separate lump within the thryoid gland, reports EndocrineWeb.com. Unlike other causes of thyroid swelling, it typically appears as an enlargement on just one side of the thyroid, reports MayoClinic.com. The lump is hard and may grow quickly. It can cause hoarseness and difficult swallowing. The lymph nodes near the neck or jaw may be swollen too.