Most thyroid nodules don't cause signs or symptoms. Occasionally, however, some nodules become so large that they can:

• Be felt
• Be seen, often as a swelling at the base of your neck
• Press on your windpipe or esophagus, causing shortness of breath or difficulty swallowing

In some cases, thyroid nodules produce additional thyroxine, a hormone secreted by your thyroid gland. The extra thyroxine can cause symptoms of hyperthyroidism such as:

• Unexplained weight loss
• Increased perspiration
• Tremor
• Nervousness
• Rapid or irregular heartbeat

A few thyroid nodules are cancerous (malignant), but determining which nodules are malignant can't be done by symptoms alone. Most cancerous thyroid nodules are slow growing and may be small when they're discovered. Aggressive thyroid cancers are rare, but these nodules may be large, firm, fixed and rapid growing.

Several conditions can cause nodules to develop in your thyroid gland:

• Iodine deficiency. Lack of iodine in your diet can sometimes cause your thyroid gland to develop thyroid nodules. But iodine deficiency is uncommon in the United States, where iodine is routinely added to table salt and other foods.
• Overgrowth of normal thyroid tissue. Why this occurs isn't clear, but such a growth — which is sometimes referred to as a thyroid adenoma — is noncancerous and isn't considered serious unless it causes bothersome symptoms from its size.

Some thyroid adenomas (autonomous or hyperfunctioning thyroid nodules) produce thyroid hormones outside of your pituitary gland's normal regulatory influence, leading to an overproduction of thyroid hormones (hyperthyroidism).

• Thyroid cyst. Fluid-filled cavities (cysts) in the thyroid most commonly result from degenerating thyroid adenomas. Often, solid components are mixed with fluid in thyroid cysts. Cysts are usually benign, but they occasionally contain malignant solid components.
• Chronic inflammation of the thyroid (thyroiditis). Hashimoto's disease, a thyroid disorder, can cause thyroid inflammation resulting in nodular enlargement. This often is associated with reduced thyroid gland activity (hypothyroidism).
• Multinodular goiter. "Goiter" is a term used to describe any enlargement of the thyroid gland, which can be caused by iodine deficiency or a thyroid disorder. A multinodular goiter contains multiple distinct nodules within the goiter, but its cause is less clear.
• Thyroid cancer. Although the chances that a nodule is malignant are small, certain factors increase your risk of thyroid cancer, such as a family history of thyroid or other endocrine cancers. Other risk factors include being younger than 30 or older than 60, being a male, or having a history of radiation exposure, particularly to the head and neck. A nodule that is large and hard or causes pain or discomfort is more worrisome in terms of malignancy.

Complications associated with thyroid nodules include:

• Problems swallowing or breathing. Large nodules or a multinodular goiter — an enlargement of the thyroid gland containing several distinct nodules — can interfere with swallowing or breathing.
• Hyperthyroidism. Problems can occur when a nodule or goiter produces thyroid hormone, leading to hyperthyroidism. Hyperthyroidism can result in weight loss, muscle weakness, heat intolerance, and anxiousness or irritability.

Potential complications of hyperthyroidism include an irregular heartbeat (atrial fibrillation); weak bones (osteoporosis); and thyrotoxic crisis, a sudden and potentially life-threatening intensification of signs and symptoms that requires immediate medical care.

• Problems associated with thyroid cancer. If a thyroid nodule is cancerous, surgery is usually required. Generally, most or all of your thyroid gland is removed, after which you'll need to take thyroid hormone replacement therapy for the rest of your life.