Thyroid Nodules: Ultrasound Features, Biopsy, and Cancer Risk

A scan for something else showed a "thyroid nodule". Or your doctor felt a lump in your neck during a checkup. The word nodule can sound scary, and the next step may be a biopsy. Here is the reassuring part: thyroid nodules are very common, and most are not cancer. This guide explains how doctors decide which nodules need a closer look.

What a thyroid nodule is

The thyroid is a butterfly-shaped gland at the front of your neck. It makes hormones that control how your body uses energy. A nodule is a lump or growth inside the thyroid. It can be solid, filled with fluid (a cyst), or both. [4]

How common they are

• About 5 in 100 women and 1 in 100 men have a nodule that can be felt. [1]
• When doctors use ultrasound, they find nodules in a large share of adults, from about 1 in 5 to more than half, depending on age and the study. [1]
• About 7 to 15 in 100 nodules turn out to be cancer. [1]

Most nodules cause no symptoms. Large ones can cause a visible lump, pressure, or trouble swallowing. [4]

What your doctor looks at

Step 1: TSH blood test

TSH (thyroid stimulating hormone) shows how well your thyroid is working. If TSH is low, the nodule may be making extra hormone. This is called a "hot" or hyperfunctioning nodule. Your doctor may order a thyroid scan with a small dose of radioactive tracer. Hot nodules are rarely cancer, and they usually do not need a biopsy. [1]

Step 2: Thyroid ultrasound

Ultrasound is the key test. It shows the size and features of each nodule. Features that raise concern include: [1] [2]

• Solid tissue, rather than fluid.
• Darker than normal thyroid tissue on the scan (hypoechoic).
• Irregular or spiky edges.
• Taller than wide in shape.
• Tiny bright spots called microcalcifications.
• Growth outside the thyroid, or abnormal lymph nodes in the neck.

Features that are reassuring include a purely fluid-filled cyst, or a spongy look with many tiny cysts. [1] [2]

Scoring systems

Radiologists often score nodules with a system. The American College of Radiology TI-RADS system gives points for each feature and sorts nodules into five levels, from TR1 (benign) to TR5 (highly suspicious). [2] The American Thyroid Association uses a similar pattern-based system, from very low to high suspicion. [1]

When a biopsy is recommended

The decision depends on both the score and the size. Under ACR TI-RADS: [2]

• TR1 and TR2: no biopsy needed.
• TR3 (mildly suspicious): biopsy if 2.5 cm or larger.
• TR4 (moderately suspicious): biopsy if 1.5 cm or larger.
• TR5 (highly suspicious): biopsy if 1 cm or larger.

Smaller nodules in TR3 to TR5 are often followed with repeat ultrasounds. The American Thyroid Association guideline uses a 1 cm threshold for high and intermediate suspicion nodules, and larger cutoffs for lower suspicion. [1] Clinicians in Canada commonly use these same US-based systems.

The biopsy

A fine needle aspiration (FNA) biopsy uses a thin needle, guided by ultrasound, to take cells from the nodule. It is done in a clinic and takes a few minutes. [1]

Bethesda results

The results are reported in one of six Bethesda categories. Each has a different estimated chance of cancer. The numbers below are rough averages from the 2023 Bethesda system. [3]

• I, nondiagnostic: not enough cells. The biopsy is usually repeated.
• II, benign: low risk, roughly 4 in 100. Usually followed with ultrasound.
• III, atypia of undetermined significance: roughly 1 in 5. Options include a repeat biopsy or molecular testing.
• IV, follicular neoplasm: roughly 3 in 10. Molecular testing or surgery to remove part of the thyroid is often discussed.
• V, suspicious for malignancy: roughly 3 in 4. Surgery is usually advised.
• VI, malignant: almost all are cancer. Surgery or, for some small cancers, close monitoring is discussed.

Molecular testing

For category III and IV results, the lab can test the cells for gene changes. These tests can help sort out which nodules are likely benign, which may help some people avoid surgery. [1] Access and coverage differ by province and by US insurance plan.

If it is cancer

Most thyroid cancers found this way are papillary thyroid cancer. It usually grows slowly and has an excellent outlook. Treatment may include removing part or all of the thyroid. For some very small, low risk cancers, active surveillance with regular ultrasound is an option. [1] [5]

Questions to ask your clinician

• What is my TSH level, and does it change the next step?
• What are the ultrasound features and the TI-RADS score of my nodule?
• Do I need a biopsy now, or follow-up ultrasounds?
• What Bethesda category did my biopsy show, and what does it mean?
• Would molecular testing help in my case?
• How often should I have repeat ultrasounds?
• Should I see an endocrinologist or a thyroid surgeon?

When to get care quickly

Most nodules are not urgent. Get care quickly if you have:

• Trouble breathing or noisy breathing.
• A neck lump that grows fast over days or weeks.
• Trouble swallowing that is getting worse.
• A new hoarse voice that does not go away.

Call 911 or go to the emergency department for severe trouble breathing.

Bottom line

Thyroid nodules are common, and most are not cancer. A TSH test and an ultrasound come first. The ultrasound features and the size decide whether you need a biopsy. If you do, the Bethesda category tells you the chance of cancer and the next step. Many nodules only need watching.

Primary AI can show you the guideline and study sources behind this topic in plain language, so you can bring better questions to your appointment.