Hyperthyroidism is the most common hormonal disease in older cats. Depending on the study, it affects up to about one in ten cats over 10 years of age. The overactive thyroid gland produces too much thyroid hormone (T4), which speeds up the metabolism and strains many organs, especially the heart and kidneys. Left untreated, the disease leads to serious complications. With treatment and regular check-ups, the outlook is usually good, although kidney disease found alongside it can affect the long-term prognosis.
Symptoms
The most common signs are weight loss despite a good or increased appetite, increased drinking and urination, vomiting, diarrhea, hyperactivity or restlessness, increased vocalization, an unkempt coat and a rapid heart rate. Many cats also lose muscle, and some show behavioral changes such as irritability. A minority of cats are instead quiet and eat poorly. Signs usually develop slowly, and owners may mistake them for normal aging.
Diagnosis
Diagnosis is based on a blood test measuring the thyroid hormone thyroxine (total T4), which we can run in our own clinic laboratory. A raised T4 together with typical signs confirms the diagnosis. In early or mild cases, or when the cat has another illness at the same time, T4 can still be within the normal range. In that case, the test is repeated a couple of weeks or more later, or free T4 is measured together with total T4. Free T4 is never interpreted on its own, because it can be falsely high in cats with other illnesses. If doubt remains, further tests such as a T3 suppression test or thyroid scintigraphy can help. During the examination, the vet may feel an enlarged thyroid gland in the neck. Blood pressure, the heart, kidney values and a urine sample are checked at the same time.
Complications
Untreated hyperthyroidism makes the heart work harder and can thicken the heart muscle, in severe cases leading to heart failure. These changes can improve once the thyroid is under control, but some cats also have a separate heart disease that does not resolve. Up to about one in four hyperthyroid cats has high blood pressure. It can damage the eyes (in severe cases causing sudden blindness), kidneys, heart and brain. Blood pressure can stay high, or even rise for the first time, after the thyroid is treated, so it is checked before and during treatment. There is also an important link with kidney disease: hyperthyroidism increases blood flow through the kidneys and their filtration rate, and muscle loss lowers creatinine, a kidney value. Together, these can hide existing kidney disease that only shows up once the thyroid is treated — in one large study, this happened in about one in six cats. Leaving the thyroid untreated does not protect the kidneys; over time it damages them further. Over-treatment, where the thyroid hormone level falls too low, can also strain the kidneys. That is why kidney values are monitored closely after treatment begins.
Treatment Options
There are four treatment options: daily medication, surgery, radioactive iodine and an iodine-restricted diet. We will help you choose the best option for your cat’s situation.
Daily medication
The medicine, thiamazole (also known as methimazole), is given every day as tablets (e.g., Felimazole) or an oral solution (e.g., Apelka). It is relatively inexpensive, but the treatment is lifelong. Most cats tolerate it well. Side effects usually appear in the first weeks and seldom after two to three months. They include vomiting, poor appetite, lethargy or severe itching of the head and neck that can cause scratch wounds. Rarely, the liver or blood cells are affected, so contact us if your cat becomes unwell or feverish, turns yellowish or bleeds easily. T4, kidney values and blood counts are checked about 3, 6, 10 and 20 weeks after starting and after every dose change, then every few months.
Handling the medicine safely at home
Thiamazole may harm an unborn child. Wash your hands after giving the medicine and after handling the cat’s litter or vomit, and do not split or crush the tablets. Women who could become pregnant should wear disposable gloves when giving the medicine and handling litter or vomit. If you are pregnant, may be pregnant or are planning a pregnancy, let someone else give the medicine and clean the litter tray.
Surgery
Surgery (thyroidectomy) can cure the disease. In about 70 % of cats, both thyroid lobes are affected, and removing both can damage the nearby parathyroid glands and cause low blood calcium, which may be temporary or permanent. The cat is first stabilized with medication, because anesthesia carries extra risk in an untreated hyperthyroid cat.
Radioactive iodine
Radioactive iodine (I-131) is considered the treatment of choice for most cats: a single treatment cures more than 95 %. In Finland, it is offered at only a couple of specialized clinics. The cat stays in isolation in the hospital for several days. For about two weeks after returning home, it must be kept indoors, and children and pregnant women must not be in contact with it. The treating clinic gives detailed instructions, including how to handle the cat’s feces. Before surgery or radioactive iodine, a trial period on medication can help predict how the kidneys will cope once the thyroid is controlled.
Iodine-restricted diet
This prescription diet (Hill’s y/d) works only if the cat eats nothing else — not even treats, other pets’ food or prey — and even then not in every cat. Some cats also refuse it because of the taste. It is therefore suitable mainly for indoor cats in single-cat homes.
Sources
- Carney H.C. ym. (2016): 2016 AAFP Guidelines for the Management of Feline Hyperthyroidism. Journal of Feline Medicine and Surgery 18(5):400–416. doi.org
- Peterson M.E. (2013): More than just T4: Diagnostic testing for hyperthyroidism in cats. Journal of Feline Medicine and Surgery 15(9):765–777. doi.org
- Peterson M.E. (2012): Hyperthyroidism in cats: what’s causing this epidemic of thyroid disease and can we prevent it? Journal of Feline Medicine and Surgery 14(11):804–818. doi.org
- Peterson M.E., Varela F.V., Rishniw M., Polzin D.J. (2018): Evaluation of Serum Symmetric Dimethylarginine Concentration as a Marker for Masked Chronic Kidney Disease in Cats With Hyperthyroidism. Journal of Veterinary Internal Medicine 32(1):295–304. doi.org
- Stammeleer L. ym. (2024): Blood pressure in hyperthyroid cats before and after radioiodine treatment. Journal of Veterinary Internal Medicine 38(3):1359–1369. doi.org
- Acierno M.J. ym. (2018): ACVIM consensus statement: Guidelines for the identification, evaluation, and management of systemic hypertension in dogs and cats. Journal of Veterinary Internal Medicine 32(6):1803–1822. doi.org
- Fimea: Felimazole Vet 2,5 mg päällystetyt tabletit kissalle, valmisteyhteenveto. fimea.fi
- Pharmaca Fennica: Apelka vet 5 mg/ml oraaliliuos kissoille, pakkausseloste. pharmacafennica.fi
- Cornell Feline Health Center: Hyperthyroidism in Cats. vet.cornell.edu
- International Cat Care: Hyperthyroidism in cats. icatcare.org