Are thyroid nodules cancer?

Thyroid nodules are common but usually are not cancer. A thyroid nodule is an abnormal growth of thyroid cells that may be solid or fluid-filled. A doctor may find one during a routine medical exam.

Thyroid nodules usually don't cause symptoms or need treatment. Only a small number of thyroid nodules are diagnosed as cancer.

The word "common" understates it. About 5% to 7% of adults have a nodule a doctor can feel by hand. Ultrasound finds far more, turning up nodules in 20% to 76% of the same adults, depending on the study. Of all nodules found, only about 4% to 6.5% prove to be cancer. More than nine in ten are harmless lumps that will never cause any trouble.

What the workup is looking for

When a thyroid nodule is found, an ultrasound of the thyroid and a fine-needle biopsy are often done to check for signs of cancer. A blood test of thyroid hormone levels is usually part of it too.

The ultrasound is more than a picture. It is a risk score. Radiologists look for features tied to cancer: tiny bright specks called microcalcifications, ragged edges, a darker look than the tissue around it, a shape taller than it is wide, and heavy blood flow.

Size then decides the next move. In general, a solid nodule with one suspicious feature gets a needle biopsy once it reaches 1 centimeter. Smaller ones are biopsied when there is more than one suspicious feature, a swollen lymph node in the neck, or a high-risk history such as past radiation to the neck.

Biopsy results are sorted into six groups under a scheme called the Bethesda system. Each group carries its own cancer risk. Nondiagnostic runs 5% to 10%. Benign is 0% to 3%. Atypia of undetermined significance is 10% to 30%. Follicular neoplasm is 25% to 40%. Suspicious for malignancy is 50% to 75%. Malignant is 97% to 99%. Ask which group your result fell into, because "the biopsy was fine" covers a wide range.

Why no one screens the whole population

The US Preventive Services Task Force recommends against screening for thyroid cancer in adults who have no symptoms. That is a grade D recommendation, meaning the harms outweigh the benefits.

The problem is overdiagnosis. Thyroid cancer detection has climbed about 4.5% per year over a decade, faster than any other cancer, with no matching drop in deaths. In South Korea, heavy use of screening ultrasound drove diagnoses to 15 times the 1993 rate while the death rate stayed flat. Autopsy studies show why: 11.5% of thyroid glands examined after death from other causes held a papillary carcinoma that never caused the person a problem.

So a nodule found by chance is not an emergency, and asking for a neck ultrasound "just to check" is not recommended.

Some symptoms do deserve prompt attention. Tell your doctor about a neck lump that is growing, trouble swallowing or breathing, pain when you swallow, or a hoarse voice that does not get better. Nodules can grow large enough to press on the windpipe or the food pipe.

Your healthcare team can explain which tests are right for you and what the results mean. If your nodule is being watched rather than biopsied, two questions are useful. How much would it have to grow to change the plan? And when is the next ultrasound due?