Hyperthyroidism/Cats

The thyroid gland is a butterfly shaped gland the lies on either side of the trachea (windpipe) and is vital in your pet’s metabolic function. Hyperthyroidism is the overproduction of thyroid hormone; this is often a disease of older cats. In cats the gland often enlarges due to the overproduction of hormones, forming nodules which are often benign (non-cancerous). Many organs are affected by hyperthyroidism, especially the heart.

Clinical Signs

The typical cat is middle aged to older, with 12 years old being the average age. The most common clinical sign of hyperthyroidism is weight loss despite the increased appetite. Cats experiencing an increase in hormone levels may show signs of restlessness and aggression.  Hyperthyroid cats often exhibit increased vocalization, particularly during nighttime hours. They may also experience periodic episodes of vomiting or diarrhea, and their fur may appear unkempt. As the disease progresses, some cats may develop anorexia.

Complications of hyperthyroidism

Hypertension (high blood pressure) this occurs due to the increased pumping pressure and increased heart rate.  Around 20% of cats with hyperthyroidism develop high blood pressure. This can lead to retinal bleeding or detachment, causing sudden blindness. Immediate treatment is necessary to prevent permanent blindness.

Thyrotoxic cardiomyopathy (disease of the heart muscle) can occur as the heart enlarges to meet higher metabolic demands, sometimes causing a heart murmur. Both cardiomyopathy and hypertension may be reversible with proper treatment.

Hyperthyroidism can lead to kidney disease due to hypertension's harmful effects, even though many cats with hyperthyroidism may already have kidney issues.

Evaluation/Diagnosis

Your primary care veterinarian will likely want to run a total thyroxine (T4) to determine if your cat is hyperthyroid. If a cat suspected of hyperthyroidism has a TT4 level in the upper normal range, a second test like free T4 by equilibrium dialysis (FT4 by ED) can be done. If this is not diagnostic referral to a veterinary specialist may be indicated.

Treatment options

When choosing treatment options there are many factors to consider and discuss with your primary care veterinarian. Thoracic (chest) radiographs and/or an echocardiogram may be recommended if your cat is found to have a heart murmur. Full blood work including chemistry/CBC and urinalysis may be recommended to determine if your cat has any underlying kidney disease.

Treatment options include oral medication, radioactive iodine, surgery, and prescription food. Depending on the treatment chosen for your cat will likely affect the following up monitoring needed in the short- and long-term interim.