This tumour type is located at the thyroid gland.

Behavior

In the thyroid, cells can degenerate and form a tumour. The thyroid is responsible for the production of thyroid hormones who play an important part in the metabolism. Usually, benign thyroid tumours are very small and do not cause abnormalities of the metabolism. When one can feel a lump at the thyroid and/or notice symptoms of a disturbed metabolism, there is a big chance it concerns a malignant tumour.

The original (primary) tumour will originate at the thyroid and in 60% of patients, both thyroid lobes are involved. If both lobes are affected, there are greater chances for metastases. Furthermore, the larger the tumour is, the higher the chances are for metastases. Up to 35-40% has metastases at diagnosis and eventually up to 80% of these tumours will lead to metastases (towards the lungs and nearby lymph nodes).

Prevalence

These tumours are most prevalent in older dogs, with a median age of 9-11 years.

Breed predisposition

Thyroid tumours are more often found in Beagles, Boxers, Golden retrievers and Siberian Huskies.

Symptoms

Symptoms can consist of a lump in the neck (which can lead to rapid breathing, coughing, difficulties swallowing, change of bark tone), but also a too low or too high function of the thyroid. When the function is too low, the dog can be languid, have a thinner undercoat and gain weight. When the function is too high, the dog can eat a lot and still lose weight, be hyperactive, drink and urinate a lot and/or have diarrhea.

Obtaining a tissue sample is often tricky as there are many blood vessels in the vicinity of the thyroid gland and the gland itself is also well vascularised. Standard staging is recommended. Via blood analysis (including the measurement of the thyroid hormone thyroxin), urine analysis, X-rays of the lungs and tissue samples of nearby lymph nodes, the veterinarian can estimate the tumour’s behaviour and the most appropriate approach. High thyroxin values can indicate a deviation of the thyroid function, however functional thyroid tumours are only diagnosed in 0-30% of the cases (depending on the study). The blood and urine analysis indicate how well other organs such as the heart, kidneys and liver are functioning. Via ultrasound or CT scan of the neck, the veterinarian can confirm whether the tissue mass originates from the thyroid, how far this tumour spreads and how perfused it is. Scintigraphy can also show whether there are metastases present, where they are situated and indicate if the tumour can be treated via radioactive iodine.

The treatment choice depends on the size of the tumour, its invasion of surrounding tissues, the presence of metastases and whether there are symptoms present of a disturbed thyroid function.

Surgery

Surgery is advised when the tumour is not attached to the surrounding tissues (which is the case in 25-50% of thyroid tumours) and there are no metastases. Risks linked to surgery are possible bleedings, accidentally removing the nearby parathyroid glands (causing a disturbance of the calcium metabolism), larynx nerve damage and a reduced thyroid function when both thyroid lobes have to be removed.

Radiotherapy

Radiation is often used after surgery when residual tumour tissue is suspected or when surgery is not a good option (e.g. when the tumour is fused to the environment). Possible side effects of radiation consist of damage to the larynx, trachea and oesophagus, although in general, radiation is well tolerated. It is possible that after a while (months to years after treatment) insufficient thyroid hormones are produced. On the other hand, these hormones can be supplemented.

Radioactive iodine therapy

If the tumour absorbs enough iodine, radioactive iodine therapy is an interesting treatment option. Radioactive iodine is administered to the dog and will build up in the thyroid tissue. There, it locally radiates the thyroid tissue, including the tumour. Depending on the amount of thyroid tissue that gets destroyed, it might be necessary to supplement thyroid hormones after the treatment. The advantage of this treatment option vs radiation therapy is that radioactive iodine targets both the metastases of the thyroid tumour as well as the primary thyroid tumour very precisely (if both take up iodine). Moreover, if necessary, the treatment can be repeated without much side effects. Treatment of thyroid carcinoma using 131 I can effectively alleviate clinical signs and reduce disease burden in a proportion of dogs. A study performed in 2022 showed that improvement of clinical signs was seen in 76.2% of dogs (32/42) and that the overall response rate was 35.3% (four complete and eight partial responses).

Chemotherapy

Chemotherapy can be considered for dogs with large, non-resectable tumours or numerous metastases, although the results for chemotherapy so far have not been as succesful as the treatments mentioned above. In some dogs with this tumour type good results were obtained (up to a year survival), but at this point it is unclear which dogs benefit from this.

Targeted therapy

In addition, off-label use of toceranib, a tyrosine kinase inhibitor has been described.

Overall, the prognosis is very good after surgery (median survival time of 3 years if the tumour is loose (not attached to the surrounding tissue), 6-12 months when spread in the surrounding tissues). For radiation and radioactive iodine therapy good results were obtained as well.

For dogs in whom the thyroid tumour could not be removed surgically and as a result received radiation therapy, 80% was still alive after 1 year and 72% after 2 years. The median duration of survival for radiation varies between 24.5 to up to more than 45 months.

According to a study with 43 dogs, the median survival times after radioactive iodine therapy was 30 (when limited to radioactive iodine therapy) to 34 (when radioactive iodine therapy was combined with surgery) months. In another study with 39 dogs, median survival time varied between 12 months (when metastases were present) and 2.3 years (when no metastases were present).

The composition of the tumour tissue and the tumour size have an influence on the prognosis. The higher the tumour grade and size, the worse the prognosis is. For malignant tumours with a size between 21-100 cm3, metastases were reported in 74% of cases, for tumours larger than 100 cm3 this was 100%. When caught early and when the tumour isn’t attached to the surrounding tissues, the prognosis is good.