Thyroid nodules are common, discrete lumps within the thyroid gland, found in a large portion of the adult population, especially with routine imaging. The vast majority of these growths, estimated at over \(90\%\), are benign, meaning they are non-cancerous. This article addresses whether these non-malignant thyroid nodules tend to increase in size over time.
The Typical Trajectory of Benign Nodule Growth
The answer to whether benign nodules grow is generally yes, but the process is highly variable and often quite slow. A significant number of nodules remain stable for many years, and some may even shrink naturally. For those that do grow, the pace is typically gradual, with the average increase being less than \(2\) millimeters per year. Growth is often intermittent, occurring in spurts rather than a continuous, steady expansion. Slow growth in a confirmed benign nodule is considered a normal part of its natural progression and is not a sign of malignancy.
When tracking size changes, volume is a more accurate metric than a single diameter measurement because growth occurs in three dimensions. Long-term studies show that even with significant growth, the chance of a previously biopsied benign nodule turning out to be malignant is very low.
Biological Factors Influencing Nodule Size
The underlying mechanisms driving changes in nodule size are related to both hormonal influence and the nodule’s internal structure. The Thyroid Stimulating Hormone (TSH), released by the pituitary gland, acts as a primary growth factor for thyroid cells. Fluctuations in TSH levels, even within the normal range, can stimulate the follicular cells to multiply, contributing to the nodule’s slow, steady growth. The internal composition of the nodule also plays a large role in sudden size changes. Many benign nodules contain cystic components, which are fluid-filled areas.
A rapid increase in size may be due to the accumulation of fluid within cystic components or a sudden hemorrhage (bleeding) into the nodule. This accumulation of fluid or blood causes a quick expansion of the nodule volume, which can be alarming but is often temporary, with the size stabilizing or even shrinking afterward.
Clinical Monitoring and Defining Significant Change
High-resolution ultrasound is the standard tool used to track size and evaluate the nodule’s characteristics over time, allowing for precise measurements necessary for longitudinal comparison. Clinical guidelines establish specific thresholds to determine if growth is significant enough to warrant further investigation, such as a repeat biopsy. Significant growth is typically defined as a \(20\%\) or greater increase in size in at least two dimensions, with a minimum increase of \(2\) millimeters, or a volume increase of \(50\%\) or more.
Monitoring is commonly performed with serial ultrasound examinations, initially spaced between six and eighteen months after the benign diagnosis. If the nodule remains stable after the initial surveillance period, the follow-up interval can often be extended to three to five years. Significant growth that crosses these established thresholds necessitates a repeat Fine Needle Aspiration (FNA) to confirm the nodule remains benign and rule out a rare missed malignancy.
Interventions for Problematic Nodule Growth
While most growing benign nodules require only continued observation, intervention becomes necessary if the growth causes problems for the patient. A nodule that grows large enough to cause compressive symptoms, such as difficulty swallowing, a feeling of pressure, or voice changes, may require treatment. Cosmetic concerns due to a visible mass in the neck can also be an indication for intervention. Minimally invasive, non-surgical options are now widely used to reduce nodule volume.
For predominantly cystic nodules, percutaneous ethanol injection (PEI) involves draining the fluid and injecting ethanol to prevent recurrence. For solid or mixed solid-cystic nodules, thermal ablation techniques, such as Radiofrequency Ablation (RFA) or Microwave Ablation (MWA), use heat to destroy the nodule tissue, causing it to shrink over time. Surgical removal, typically a hemithyroidectomy (lobectomy), is usually reserved for nodules that are extremely large, causing severe symptoms, or if non-invasive methods have failed to provide relief. Although TSH suppression therapy using thyroid hormone medication was once common to try and slow growth, it is not routinely recommended today due to variable effectiveness and potential side effects.