Thyroid cysts most often form when a solid thyroid nodule breaks down internally, causing fluid to collect inside it. This process, called cystic degeneration, accounts for the vast majority of thyroid cysts. Less commonly, cysts develop from the gradual buildup of colloid, the protein-rich fluid the thyroid uses to store hormones. Either way, thyroid cysts are overwhelmingly benign.
How Thyroid Cysts Form
The thyroid gland frequently develops small lumps of overgrown tissue called nodules. Ultrasound can detect thyroid nodules in up to 68% of the general population, and many of these nodules eventually develop cystic changes. The two main pathways are distinct.
In the most common scenario, a solid nodule outgrows its own blood supply. The tissue in the center doesn’t get enough oxygen, so it breaks down and liquefies. Fluid fills the space where solid tissue used to be, creating what’s technically called a pseudocyst because the fluid-filled cavity lacks a true lining. Over time, these pseudocysts may develop an inner lining, but they start as a consequence of tissue breakdown rather than as a purpose-built structure.
The second pathway involves colloid cysts. Colloid is the gel-like substance your thyroid produces to store hormones. When follicles (the tiny sacs inside the thyroid) accumulate too much colloid or dilate abnormally, they can expand into large, fluid-filled cysts. These are considered “true” cysts because they’re lined with a layer of cells from the start, though they’re rare, representing only about 4% of all thyroid cysts.
Nodule Degeneration: The Primary Cause
Most thyroid cysts in adults result from the acute or gradual breakdown of benign thyroid adenomas. An adenoma is simply an overgrowth of thyroid tissue. Why some people develop adenomas isn’t fully understood, but the growths themselves aren’t cancerous and aren’t considered dangerous unless they grow large enough to press on nearby structures.
Three specific processes drive the breakdown of these nodules into cysts:
- Tissue overgrowth outpacing blood supply. As a nodule grows, its interior can lose adequate circulation. Without enough blood flow, the tissue dies and liquefies.
- Hemorrhage. Bleeding into a nodule can happen spontaneously, rapidly filling it with blood and fluid. Larger nodules are more prone to this kind of internal bleeding. The sudden expansion can cause noticeable neck pain or swelling that appears over hours or days.
- Immune-related damage. In some cases, the body’s immune response damages nodule tissue, leading to breakdown and fluid accumulation. This can occur alongside conditions like Hashimoto’s thyroiditis, where the immune system attacks the thyroid.
The Role of Iodine Deficiency
Iodine is the raw material your thyroid needs to produce hormones. When iodine intake is low, the gland works harder to compensate, often growing larger and developing nodules in the process. In populations with mild to moderate iodine deficiency, this chronic overstimulation increases the prevalence of nodular goiter, which is an enlarged thyroid studded with nodules. Over time, those nodules can undergo the same degeneration process that produces cysts.
In countries with iodine-fortified salt, this cause is less common, but it remains a significant driver of thyroid cysts in parts of the world where iodine intake is still inadequate.
Simple Cysts vs. Complex Cysts
Not all thyroid cysts look the same on imaging, and the distinction matters because it affects how likely the cyst is to be problematic.
Simple cysts contain only fluid. They appear as dark, uniform circles on ultrasound and have a very low probability of being cancerous. Most are colloid cysts or fully liquefied nodules, and they often require no treatment at all.
Complex (or mixed) cysts contain both fluid and solid tissue. These are far more common than purely fluid-filled cysts, and their risk profile depends on the ratio of solid to fluid. When the solid portion makes up at least half of the cyst, the risk of malignancy reaches about 20%. When a large cyst has only a small solid component, that risk drops to around 5%. Features like irregular margins or small solid projections growing into the fluid cavity also raise concern.
When Cysts Cause Symptoms
Most thyroid cysts produce no symptoms at all and are discovered incidentally during imaging for something else. Symptoms typically depend on size. In studies comparing symptomatic and asymptomatic patients, those experiencing compression symptoms had nodules averaging 3.8 cm, compared to 2.2 cm in people without symptoms.
When cysts do grow large enough to cause trouble, the most common complaints are a visible lump in the neck, difficulty swallowing, a sensation of pressure or fullness in the throat, and occasionally voice changes if the cyst presses on the nerve that controls the vocal cords. A sudden increase in size, often from hemorrhage into the cyst, can cause sharp pain and rapid swelling. Among patients who ultimately had surgery for compressive symptoms, over 92% reported improvement afterward.
How Cysts Are Evaluated
Ultrasound is the standard first step. It reveals whether a lump is purely fluid-filled, mostly solid, or a mix of both. The ultrasound appearance guides the next decision: whether a sample needs to be taken with a fine-needle aspiration biopsy.
Guidelines from major radiology and endocrinology organizations generally recommend biopsy for mixed solid-and-cystic nodules at 2 cm or larger, or at smaller sizes if the nodule has worrisome features like irregular margins, tiny calcifications, or a shape that’s taller than it is wide. Purely cystic nodules without suspicious features are typically monitored rather than biopsied. When an aspiration is performed on a cyst, it serves double duty: it removes fluid (which can relieve symptoms) and allows the fluid and any solid material to be examined under a microscope.
Some cysts refill after drainage. Recurrent cysts that keep causing symptoms or that have concerning features on repeat evaluation may be treated with additional procedures, ranging from repeat aspiration to surgical removal, depending on the specifics.