A swelling in the neck due to an enlarged thyroid gland is called a goitre. Worldwide, the most common cause of thyroid goiter is iodine deficiency.
However, this is becoming a rarer cause because iodine is now present in drinking water and some of the foods we eat, such as table salt. Hashimotos thyroiditis and Graves’ disease, two autoimmune diseases, are much more common causes of goiter in the western world.
Nodules are lumps in the thyroid gland. Thyroid nodules are common and occur in 50 per cent of adults. Most nodules don’t cause any signs or symptoms.
Some nodules can become so large that they can be felt or seen, or can press on the windpipe or oesophagus, causing shortness of breath or difficulty swallowing. In some cases, thyroid nodules produce additional thyroxine — a hormone secreted by the thyroid gland. The extra thyroxine can cause symptoms of hyperthyroidism such as:
Unexplained weight loss;
Intolerance to heat;
Rapid or irregular heartbeat.
A few thyroid nodules (seven per cent) are cancerous, but determining which nodules are malignant can’t be done by symptoms alone.
Risk factors for thyroid carcinoma include age of less than 20 years or more than 60 years, a history of neck irradiation and a family history of thyroid cancer.
The imaging modality of choice for the investigation of thyroid nodules is high-resolution ultrasound. The patient’s family physician or ENT surgeon will therefore refer patients to a radiologist for ultrasound assessment of an enlarged thyroid gland. The job of the radiologist is to determine which thyroid nodules are suspicious in appearance and need to undergo a biopsy procedure called Fine Needle Aspiration Cytology (FNAC) to check for cancerous cells.
The ultrasound features suggestive of malignancy are as follows:
Thyroid calcifications can be classified as microcalcification, coarse calcification or peripheral calcification.
Microcalcifications are one of the most specific features of thyroid malignancy. They appear as tiny white spots without shadowing on ultrasound and are most commonly found in papillary thyroid cancer. They can occur in other types of thyroid cancer as well as benign conditions.
Coarse calcification may occur with microcalcification in papillary carcinoma and they are the most common type of calcification in medullary thyroid carcinomas.
Coarse calcification causes shadowing on ultrasound.
Peripheral calcification is most commonly seen in multinodular goiter, but may also be seen in malignancy.
LOCAL INVASION AND LYMPH NODE METASTASIS
Direct tumour invasion of adjacent soft tissue and mets to lymph nodes are very specific signs of thyroid malignancy. There may be subtle extension of the tumour beyond the contours of the thyroid gland or frank invasion of adjacent structures. Clinical features include shortness of breath, hoarseness and difficulty swallowing, depending on the neck structure invaded.
Abnormal appearing lymph nodes on the same side as the thyroid tumour should arouse suspicions of metastasis.
Margins, contour and shape.
Benign nodules often have a complete hypoechoic (dark) halo around them. A thyroid nodule is considered irregular when less than 50 per cent of its margin is well demarcated. Irregular margins can suggest cancer with early spread. A rounded nodule that is higher than wide is also suspicious.
Blood flow in the central part of the tumour that is greater than that in the surrounding thyroid tissue is the most common pattern in thyroid malignancy. A nodule without any blood flow at all is very unlikely to be cancerous.
Very dark (hypoechoic) nodules are more likely to be cancerous.
After identifying a suspicious nodule, the radiologist will then perform an ultrasound-guided fine needle aspiration to retrieve cells, which are then examined by a cytologist. If a diagnosis of thyroid cancer is made, the radiologist will once again be called upon to stage the cancer. X-ray, ultrasound, CT, MRI and nuclear medicine imagine may all be used to help to define the extent of cancer spread.