Hypoechoic nodules are nodules that look darker on an ultrasound. They’re usually benign, but your healthcare professional may want to do some followup tests to be sure.

Thyroid nodules are small lumps or bumps in your thyroid gland, which is located at the base of your neck. They’re small and usually only show up during and exam. Nodules are different from an enlarged thyroid, also called a goiter, but the two conditions do sometimes coexist in the case of a nodular goiter.

The term “hypoechoic” refers to the way a nodule looks on an ultrasound, also called a sonogram. Ultrasound machines produce sound waves that penetrate your body, bouncing off tissues, bones, muscles, and other substances.

The way that these sounds bounce back to form an image is known as echogenicity. Something with low echogenicity appears dark in the image and is called hypoechoic, while something with high echogenicity looks light and is called hyperechoic.

A hypoechoic nodule, sometimes called a hypoechoic lesion, on the thyroid is a mass that appears darker on the ultrasound than the surrounding tissue. This often indicates that a nodule is full of solid, rather than liquid, components.

Most thyroid nodules are benign, which means they aren’t cancerous. About 2 or 3 in 20 are malignant, or cancerous. Malignant nodules can spread to surrounding tissues and other parts of the body.

Solid nodules in your thyroid are more likely to be malignant than fluid-filled nodules, but they’re still rarely cancerous.

Keep in mind that, while hypoechoic nodules are more likely to be cancerous, echogenicity itself isn’t a reliable predictor of thyroid cancer. It’s simply a sign that your doctor may need to do additional testing, such as a biopsy.

Thyroid nodules are extremely common. Some studies suggest that more than 50 percent of the population may have a thyroid nodule.

Thyroid nodules can be caused by a variety of things, including:

If a hypoechoic nodule shows up on your ultrasound, your doctor will likely do some additional testing to figure out what’s causing it.

Additional tests include:

  • Fine needle aspiration (FNA) biopsy. This is a simple in-office procedure that only takes about 20 minutes. During an FNA, your doctor inserts a thin needle into the nodule and removes a tissue sample. They may use an ultrasound to guide them to the nodule. Once the sample is collected, it will be sent to a lab for testing.
  • Blood test. Your doctor may do a blood test to check your hormone levels, which can indicate whether your thyroid is working properly.
  • Thyroid scan. This imaging test involves injecting the area around your thyroid with a radioactive iodine solution. You’ll then be asked to lie down while a special camera takes pictures. How your thyroid appears in these images can also give your doctor a better idea of your thyroid function.

Thyroid nodules are very common and benign in most cases. If your doctor found a hypoechoic nodule during an ultrasound, they may simply do some additional testing to make sure there’s no underlying cause that needs treatment. While thyroid nodules could be a sign of cancer, it isn’t likely.