Hyperthyroidism is the most common endocrine disorder that affects older cats and a common problem overall. Ten percent of cats older than 10 years will develop this disease. In hyperthyroidism, the thyroid gland becomes enlarged and overproduces thyroid hormones, which alter a cat’s metabolism and lead to secondary organ damage. The Neighborhood Vet team knows how damaging hyperthyroidism can be for a cat, so we’re sharing more information to help pet owners recognize the early signs and take action for their pet’s benefit.

Hyperthyroidism causes

Most hyperthyroidism cases in cats are caused by a benign (i.e., non-cancerous) tumor or enlargement of the thyroid gland, causing the gland to produce more hormones. Researchers aren’t sure why so many older cats develop this disease, but diet or environmental toxins may play a role. Because the definitive cause is unknown, no specific steps are recommended for prevention except regular wellness screenings and blood tests as suggested by our team.

Hyperthyroidism signs

Excess thyroid hormones cause your cat’s metabolism to speed up, which affects many of their major organs and systems. Pet owners first may notice increased energy or a slight weight loss, which often are viewed as positive signs. When pet owners mention to our team that their cat has finally lost weight or is acting like a kitten again, we are suspicious that hyperthyroidism could be the cause. Hyperthyroidism signs in cats may include:

  • Weight loss
  • Increased appetite
  • Increased thirst or urination
  • Anxiety or hyperactivity
  • Greasy or unkempt hair coat
  • Fast heart rate
  • New heart murmurs or gallop rhythms

Hyperthyroidism increases heart rate and blood pressure, which can lead to heart or kidney damage over time. Cats sometimes may exhibit heart failure or have detached retinas and blindness from high blood pressure.

Hyperthyroidism diagnosis

Routine screening tests for the main thyroid hormone, T4, can detect elevated levels. If an initial T4 test comes back normal but we strongly suspect hyperthyroidism, we may recommend additional blood tests to confirm the diagnosis. Our veterinarians also will recommend a general blood chemistry profile, complete blood count, and urinalysis to assess overall health and organ function that could be affected by hyperthyroidism. Blood pressure tests, a chest X-ray, and a heart ultrasound (i.e., echocardiogram) are additional ways to evaluate cardiovascular function.

Hyperthyroidism treatment options

Hyperthyroidism in cats is highly treatable, with some options offering a permanent cure. Heart or kidney dysfunction or damage may improve once thyroid hormone levels are back in the normal range, or the damage may be permanent and require separate treatments or management strategies. Options for lowering thyroid hormone levels include:

  • Radioactive iodine therapy — This treatment is considered the gold standard for cats who are good candidates and usually provides a permanent cure. Cats are injected with a small amount of radioactive iodine, which the abnormal thyroid cells absorb in an attempt to make more thyroid hormone. The radioactivity destroys the abnormal cells without harming the cat and is eliminated from the body in a few days. This treatment is non-invasive and highly effective but requires special quarantine protocols and can be performed only by licensed specialty facilities.
  • Medication — Medication called methimazole can lower thyroid hormone levels, but it must be given twice daily, every day for the remainder of the cat’s life. Most cats tolerate the medication well, but side effects are possible. After starting this medication, it is imperative that we recheck your cat’s blood work within a few weeks to screen for side effects. This option is non-invasive and cost-effective, but daily medication administration may not be ideal for some pets or pet owners.
  • Iodine-deficient diet — This option is best for mild cases and single-cat households, and uses a prescription diet devoid of iodine (e.g., Hills Science Diet y/d) so the thyroid gland no longer has the ingredients needed to create excess thyroid hormone. Cats cannot be given ANY other food or treats during treatment and are best confined inside to keep them from eating insects, rodents, or grass outside.
  • Surgery — Surgery to remove the abnormal thyroid gland is chosen infrequently because it requires general anesthesia, costs more, and can damage other nearby glands that control calcium balance or cause thyroid hormones to dip too low.

The long-term outlook for hyperthyroid cats

Left untreated, many hyperthyroid cats will experience complications, and their condition will deteriorate quickly. Most respond well to treatment, but their condition must be monitored closely over time. Our team will recommend thyroid hormone testing at regular intervals and any time we make changes to your pet’s treatment plan. Blood pressure, kidney, and heart function also will be monitored closely to make treatment adjustments easier if needed. 

Cats older than 10 years are susceptible not only to hyperthyroidism, but also to many other age-related diseases that are most responsive to treatments in their earliest stages. Twice-yearly wellness visits with The Neighborhood Vet are the best way to keep your senior cat healthy and happy. Call us to schedule a house call or in-office visit, or to learn more about hyperthyroidism in cats.