Thyroid nodules are lumps of tissue that form within the thyroid gland, and they’re remarkably common. Ultrasound detects them in up to 67% of the general population, though only 4 to 7% are large enough to feel during a physical exam. Most are benign, with only 5 to 15% turning out to be cancerous. The causes range from simple tissue overgrowth to iodine deficiency, chronic inflammation, and genetic predisposition.
Overgrowth, Cysts, and Goiter
The most straightforward cause of a thyroid nodule is an overgrowth of normal thyroid tissue, called a thyroid adenoma. These are noncancerous growths where thyroid cells simply multiply more than they should. Why this happens in one spot and not the rest of the gland isn’t fully understood, but the result is a solid lump that functions independently from the surrounding tissue. Some adenomas produce thyroid hormones on their own, which can push overall hormone levels too high.
When adenomas break down over time, they can fill with fluid and become thyroid cysts. Many cysts are a mix of solid tissue and fluid rather than being purely liquid-filled. Purely cystic nodules (no solid component at all) carry less than a 1% risk of being cancerous and generally don’t need a biopsy.
A third common cause is multinodular goiter, where the thyroid gland enlarges and develops multiple nodules. This can result from iodine deficiency or from underlying thyroid dysfunction, though in many cases the exact trigger remains unclear.
Iodine Deficiency
Your thyroid needs iodine to produce hormones. When iodine intake falls short, the gland works harder and often enlarges, creating conditions ripe for nodule formation. A 2025 systematic review and meta-analysis found that people with iodine deficiency had about a 24% higher odds of developing thyroid nodules compared to those with adequate intake. Mild to moderate deficiency is also linked to both toxic and nontoxic nodular goiter.
In countries where salt is routinely iodized, severe deficiency is rare. But it still occurs in regions where iodized salt isn’t standard, and in individuals who follow very restrictive diets. Interestingly, the same meta-analysis found that excessive iodine intake didn’t show a consistent effect on nodule risk, so more isn’t necessarily better.
Hashimoto’s Thyroiditis and Inflammation
Hashimoto’s thyroiditis is an autoimmune condition where the immune system attacks the thyroid. This creates chronic inflammation: immune cells flood the gland, lymphoid follicles form, and over time the tissue develops fibrosis and begins to shrink. This ongoing cycle of damage and repair can predispose the thyroid to forming nodules. The gland essentially remodels itself under persistent immune assault, and nodules emerge as part of that disordered regrowth.
Because Hashimoto’s is the most common cause of hypothyroidism in developed countries and affects women far more often than men, it’s a significant contributor to the overall prevalence of thyroid nodules.
The Role of TSH
Thyroid-stimulating hormone, or TSH, is the signal your pituitary gland sends to tell the thyroid to produce more hormones. TSH also drives thyroid cell growth. When TSH levels run high, whether from hypothyroidism or other causes, it continuously stimulates thyroid cells to multiply. This chronic stimulation can promote both the formation and growth of nodules.
Animal studies have shown that sustained TSH overstimulation leads to thyroid tissue overgrowth and, in some cases, eventual cancer. In human studies, higher TSH levels correlate with a greater likelihood that a nodule is malignant. One study found that nodules were 1.54 times more likely to be cancerous when TSH levels were elevated. This is part of why thyroid cancer patients are often kept on medication that suppresses TSH: it slows disease progression and reduces recurrence.
Radiation Exposure
Exposure to radiation, particularly during childhood, is one of the most clearly established causes of thyroid nodules. The thyroid is unusually sensitive to radiation, and the effects can appear decades later. Data from populations exposed to nuclear fallout illustrate this starkly: 34 years after a test explosion in the Marshall Islands, 22% of exposed individuals had developed thyroid nodules, compared to just 1.5% in unexposed people from the same region.
The risk increases with higher radiation doses and decreases with older age at exposure, meaning children’s thyroids are especially vulnerable. Women exposed to radiation were 3.7 times more likely to develop a nodule than men. Medical radiation to the head and neck area during childhood, once commonly used to treat conditions like acne or enlarged tonsils, also raises the risk significantly.
Age, Sex, and Genetic Factors
Thyroid nodules become more common as you age. In large population studies, the average age of people diagnosed with nodules is around 48, compared to about 44 for those without them. Women are affected considerably more often than men, with about 60% of nodule cases occurring in women. This gender gap likely reflects the influence of estrogen and other reproductive hormones on thyroid tissue growth, though the exact mechanisms are still being studied.
Genetics also play a role. If thyroid nodules or thyroid disease runs in your family, your risk is higher. Certain inherited syndromes raise the stakes further. Cowden syndrome, caused by mutations in the PTEN gene, carries a 10 to 35% incidence of thyroid cancer and is associated with nodule formation as part of a broader pattern of tumor development affecting the breast, uterus, kidney, and skin. Familial adenomatous polyposis, a condition primarily associated with colon cancer, also increases the risk of thyroid nodules and thyroid cancer.
How Nodules Are Evaluated
Because most thyroid nodules are benign, the key question after discovery is whether a nodule needs further testing. Guidelines from the American Thyroid Association base this decision on two factors: the nodule’s size and how it looks on ultrasound.
Nodules that appear solid, dark on ultrasound, and have irregular edges, tiny calcium deposits, or a taller-than-wide shape carry the highest suspicion for cancer, with an estimated malignancy risk above 70%. These are recommended for biopsy (a fine needle aspiration) once they reach 1 cm. Solid nodules with smooth margins but a darker appearance fall into an intermediate category, with a 10 to 20% cancer risk, and are also biopsied at 1 cm. Nodules that look brighter or partially cystic without worrisome features have a lower risk (5 to 10%) and are typically biopsied at 1.5 cm. Spongy or mostly cystic nodules with a cancer risk below 3% may be watched rather than biopsied, even at 2 cm.
Autopsy data reveal that 50% of people without any known thyroid problems have nodules larger than 1 cm. This means many nodules exist silently throughout a person’s life without causing symptoms or health issues. The challenge isn’t that nodules exist; it’s distinguishing the small percentage that need attention from the majority that don’t.