Symptoms of an Overactive Thyroid in Cats

Updated November 21, 2016

Overactive thyroid, or hyperthyroidism, in cats occurs when the thyroid gland produces too much thyroid hormone. The condition is usually caused by a tumour on the thyroid gland. Hyperthyroidism has a variety of symptoms and signs vary with each individual case. One of the most common symptoms is weight loss accompanied by an increased appetite. Fortunately, the condition is very treatable when properly diagnosed.


The thyroid gland is a butterfly-shaped gland located on either side of a cat's throat. The thyroid's primary function is to produce hormones that regulate metabolism and organ function. Hyperthyroidism occurs when the thyroid gland is overactive and produces too much thyroid hormone


The most common cause of hypothyroidism is a benign tumour in the thyroid gland. This tumour secretes excess thyroid hormone, leading to hypothyroidism.


Hyperthyroidism has a variety of symptoms. Like many diseases, the symptoms vary from animal to animal. Weight loss is a typical symptom, but some cats with overactive thyroids do not lose weight. The affected cat will often show an increased appetite and demand food more frequently with no weight gain. Occasionally a decreased appetite is noted. Cats with hyperthyroidism frequently have bouts of vomiting and diarrhoea. They will also drink more water and urinate more frequently. Hypothyroidism will often cause behavioural changes as well. Affected cats will often become more frisky and energetic. Sometimes a decrease in activity or overall weakness is noticed, but it is more typical for the cat to increase its activity level. Cats with hyperthyroidism also become more vocal, meowing and howling more often than normal.


A veterinarian will typically diagnose hyperthyroidism by doing a blood test to check the cat's thyroxine (T4) levels. High T4 levels indicate hyperthyroidism.


There are three common treatments for hyperthyroidism. Antithyroid drugs such as Tapazole are the treatment of choice for many veterinarians and also the least expensive. Surgical removal of the thyroid gland removes the affected part of the thyroid and is an effective but costly treatment. Radioactive iodine treatment involves an injection of I-131 that acts to destroy the affected part of the thyroid. This treatment method is highly effective, with almost all cases returning to normal within a month of treatment. It is also the most expensive treatment option.

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