Thyroid nodules are lumps that form within the thyroid gland, and they’re remarkably common. When checked with ultrasound, a large portion of adults have at least one, though most never cause symptoms or require treatment. Between 90% and 95% of thyroid nodules are benign, meaning only a small fraction turn out to be cancerous. Understanding what causes them can help you make sense of a diagnosis that often sounds more alarming than it is.
Iodine Deficiency and Thyroid Overstimulation
One of the best-understood causes of thyroid nodules is iodine deficiency. Your thyroid needs iodine to produce its hormones, and when it doesn’t get enough, a chain reaction begins. The pituitary gland in your brain detects falling hormone levels and responds by releasing more thyroid-stimulating hormone (TSH). TSH pushes thyroid cells to work harder, absorb more iodine, and grow. Over time, this constant stimulation causes the gland to enlarge, a condition called goiter.
At first the enlargement is uniform, but eventually the growth becomes uneven, forming distinct lumps or nodules. Some of these nodules can become “autonomous,” meaning they start producing thyroid hormone on their own regardless of signals from the brain. Researchers have found that these autonomous nodules frequently contain genetic mutations that keep the TSH signaling pathway permanently switched on. In countries where iodized salt is standard, this cause is less common, but it remains a leading driver of thyroid nodules worldwide.
Benign Overgrowths and Cysts
Many thyroid nodules are simply overgrowths of normal thyroid tissue, known as adenomas. Why certain cells start multiplying faster than their neighbors isn’t entirely clear, but these growths are not cancerous and typically stay that way. They can range from barely detectable to large enough to feel in the neck.
Thyroid cysts, the fluid-filled nodules that sometimes show up on imaging, usually develop when an adenoma breaks down internally. The solid tissue degenerates and fills with fluid, creating a pocket within the gland. In many cases, nodules contain a mix of both solid tissue and fluid. Pure cysts are almost always benign, while nodules with a higher proportion of solid material may need closer evaluation.
Hashimoto’s Thyroiditis and Autoimmune Disease
Hashimoto’s thyroiditis, the most common cause of an underactive thyroid in developed countries, creates chronic inflammation in the gland. The immune system mistakenly attacks thyroid tissue, causing ongoing damage and repair cycles that can set the stage for nodule formation. Not everyone with Hashimoto’s develops nodules, but when the two conditions overlap, the picture can become more complicated.
A large study published through the American Thyroid Association found that patients with both thyroid nodules and Hashimoto’s thyroiditis had a higher rate of cancer when those nodules were biopsied and surgically removed: 23.3% compared to 15.9% in patients without the autoimmune condition. This doesn’t mean Hashimoto’s causes cancer, but it does mean nodules in people with this condition may warrant more careful monitoring and follow-up.
Radiation Exposure
Exposure to radiation during childhood is one of the strongest known risk factors for developing thyroid nodules later in life. This includes radiation therapy directed at the head, neck, or chest for childhood cancers, as well as environmental exposure from nuclear accidents. The thyroid gland in children is particularly sensitive to radiation because it’s still growing and actively dividing.
The American Thyroid Association recommends that anyone with known childhood radiation exposure be monitored for thyroid nodules and thyroid cancer for the rest of their lives. The risk doesn’t disappear after a certain number of years. Nodules that develop in this context are evaluated with the same tools used for any thyroid nodule, but the threshold for investigation is often lower given the history.
Genetics and Family History
Certain inherited conditions significantly increase the likelihood of thyroid nodules, particularly those that carry a cancer risk. Familial adenomatous polyposis and PTEN-hamartoma tumor syndrome (sometimes called Cowden syndrome) are genetic syndromes primarily associated with tumors outside the thyroid, but they also raise the risk of thyroid growths. A separate group of hereditary conditions involves familial papillary thyroid carcinoma, which can present alongside multinodular goiter.
Even outside of these defined syndromes, having a first-degree relative with thyroid nodules or thyroid cancer increases your own risk. The interplay between genetic predisposition and environmental triggers like iodine levels, diet, and lifestyle likely explains why nodules tend to cluster in families even when no single gene mutation is identified.
Age and Sex Differences
Women develop thyroid nodules far more frequently than men, a pattern consistent enough that thyroid ultrasound clinics see overwhelmingly female patients. Hormonal factors, particularly estrogen, are thought to play a role, though the exact mechanism isn’t fully established. Researchers have noted that women of childbearing age represent a particularly distinct subgroup worth studying further.
When men do develop nodules, the characteristics tend to differ. Nodules in men are more likely to be larger in volume and to have a “taller-than-wide” shape on ultrasound, a feature radiologists watch for. Men also have a higher rate of malignant or likely malignant results on biopsy (about 9% compared to 6% in women), meaning that while nodules are less common in men, they deserve careful evaluation when found. Age is another factor: nodules become increasingly common with each passing decade, and by age 60, a substantial percentage of people have at least one.
How Nodules Are Evaluated
When a nodule is discovered, doctors use ultrasound to assess its risk level based on specific visual features. The American College of Radiology’s TI-RADS system scores nodules on five characteristics: composition (solid vs. fluid), echogenicity (how bright or dark the nodule appears), the smoothness or irregularity of its edges, its shape, and the presence of bright spots called echogenic foci. Each feature adds points, and the total score determines whether the nodule falls into a low, moderate, or high suspicion category.
Higher-risk scores lead to a recommendation for fine-needle biopsy, where a thin needle extracts cells from the nodule for examination under a microscope. Lower-risk nodules may simply be monitored with periodic ultrasounds to check for growth or changing features. The size of the nodule matters too: very small nodules, even those with suspicious features, are often watched rather than biopsied because the risk of a clinically significant cancer at that size is extremely low.
Multiple Nodules and Multinodular Goiter
Some people develop not just one but several nodules, a condition called multinodular goiter. The causes mirror those behind single nodules: genetics, iodine intake, lifestyle, and environmental factors all contribute. Having multiple nodules doesn’t necessarily increase cancer risk compared to a single nodule, but each nodule with concerning features needs its own evaluation.
Multinodular goiters can grow large enough to cause compressive symptoms, including difficulty swallowing, a sensation of pressure in the neck, or changes in voice if the gland presses on nearby nerves. In other cases, one or more nodules within the goiter become autonomous and overproduce thyroid hormone, leading to hyperthyroidism with symptoms like rapid heart rate, weight loss, and anxiety.