What is a nodule?
Thyroid nodules are round or oval-shaped growths in the thyroid gland.
They are common and are not usually harmful to a person’s health. But sometimes, thyroid nodules are caused by a serious condition, such as cancer.
You may have a single nodule or multiple nodules (called a multi-nodular goitre).
The nodules may be large enough to be felt but are more commonly found on imaging (ultrasound / CT scan) and are too small to be felt.
Most thyroid nodules do not change the amount of thyroid hormone in the body. But some cause the thyroid gland to make too much thyroid hormone. This can cause symptoms.
The thyroid gland is made of small round areas filled with a protein called colloid which contains the thyroid hormones. These small round areas are called micro-follicles.
Under different environmental influences (such as iodine deficiency), the amount of TSH made by the pituitary gland fluctuates. This results in some micro-follicles growing more than others and eventually becoming nodules.
Most are what are called colloid nodules (full of protein and regular clones of thyroid cells) but some become adenomas (single clone of cells) and some cells may undergo transformation to cancer cells.
Most nodules are benign.
Overall, the chance of a nodule to be a cancer is around 7% including in a multinodular goitre.
However, the chance varies depending on factors such as the patient’s age, the size of the nodule, but most importantly its appearance on ultrasound.
Nodules are classified based on their ultrasound appearance according to the TIRADS system (TR) as follows:
| How suspicious? | Cancer Risk | Biopsy? | |
|---|---|---|---|
| TR-1 | Normal thyroid | 0.3% | No biopsy |
| TR-2 | Not suspicious | 1.5% | No biopsy |
| TR-3 | Mildly suspicious | 4.8% | Biopsy when size >25mm |
| TR-3 | Mildly suspicious | 4.8% | Biopsy when size >25mm |
| TR-4 | Moderately suspicious | 9.1% | Biopsy when size >15mm |
| TR-5 | Highly suspicious | 35% | Biopsy when size >10mm |
The TR classification and size determine which nodules are biopsied (see table above).
A thyroid biopsy is usually done with a fine needle under ultrasound guidance. It is called a fine needle aspirate biopsy (FNAB).
It involves a small injection of local anaesthetic which numbs the skin after which a fine needle is passed multiple times into the nodule to obtain enough cells to make a diagnosis.
Biopsy results are classified according to a system called Bethesda classification from 1 to 6:
| Bethesda | Findings / explanation | Cancer risk |
|---|---|---|
| 1 | Non-diagnostic. Not enough cells in the biopsy | 5-10% |
| 2 | Benign | 0-3% |
| 3 | Atypia of uncertain significance (AUS) or follicular lesion of uncertain significance (FLUS). (Basically, some unusual looking cells but not certainly suspicious). | 10-30% |
| 4 | Follicular neoplasm or suspicious for a follicular neoplasm. (Neoplasm means growth but is not necessarily a cancer). | 25-40% |
| 5 | Suspicious for malignancy. (suspicious for a cancer) | 50-75% |
| 6 | Malignant (cancer cells) | 97-99% |
Surgery depends on biopsy results and the presence or absence of symptoms (see goitre).
Bethesda 2 nodules do not need surgery.
Bethesda 1 nodules may need a repeat biopsy.
Bethesda 5&6 require surgery.
Bethesda 3&4 usually require surgery but molecular testing can be considered to guide this decision. Molecular testing is not widely available and its definitive role remains investigational.
If a nodule needs to be removed, this usually means removing the thyroid lobe (or half) that contains that nodule.
Removing the nodule only without removing the gland is not advisable as it means causing scarring making further surgery on that side very difficult with increased risk of complications such as nerve injury (see surgical complications of thyroidectomy).