Nothing is cuter than hearing your kitty meow, huh? I love the sound so much that I named my veterinary website The Meowing Vet, for crying out loud! I’m like the ultimate Feline Fangirl! However cute your cat’s meow is, though, it may be time for a visit to your veterinarian if your cat begins meowing more than usual paired with increased activity, eating and drinking more than usual yet losing weight, and heading to the litterbox more often. These signs can indicate that your cat has developed hyperthyroidism, a disorder of the thyroid gland requiring treatment. Keep reading for some Q & A on this “meowing” disease.

Q: What is the thyroid gland? Where is it located, and what is its function?

A: The thyroid gland is a small organ of the endocrine system (the organ system responsible for hormone production released into the circulatory system). The thyroid gland consists of two lobes that lie on either side of the trachea (windpipe) in the throat region of all vertebrate animals (i.e. those with a backbone, such as humans, dogs, and cats). It is commonly described as having a “butterfly” shape.

The thyroid gland produces thyroid hormones, primarily thyroxine (or T4) and triiodothyronine (or T3). These hormones are responsible for the body’s metabolism, meaning the innumerable chemical reactions occurring across cells in each tissue and organ. Thus, T3 and T4 affect the metabolism of carbohydrates, fats, and proteins across nearly every organ in the body. Additionally, thyroid hormones aid in the regulation of calcium in the body, the sleep cycle, heart rate and strength, as well as the development of a healthy pregnancy and newborn. The thyroid gland is in turn controlled by other components of the endocrine system: the hypothalamus and the pituitary gland.

Q: What is hyperthyroidism?

A: Let’s break down the word “hyperthyroidism.” Hyper means “too much” or “excessive,” and thyroid refers to the thyroid gland. Therefore, hyperthyroidism is a disease in which an overly active thyroid gland produces an excessive level of thyroid hormones. This causes the metabolic rate of the affected animal to be much higher than normal.

Q: Are dogs or cats affected by hyperthyroidism?

A: In veterinary medicine, cats are the prime patients most affected by hyperthyroidism. Dogs, on the other hand, are more commonly affected by hypothyroidism (or decreased activity of the thyroid gland, resulting in a slower metabolic rate). Both conditions require treatment to return thyroid hormones to a more normal level (or euthyroid level). However, when a cat with hyperthyroidism begins receiving medical therapy, the achievement of thyroid hormonal regulation can take awhile, and the treatment may occasionally “overshoot” its goal, causing a hyperthyroid kitty to temporarily become hypothyroid until the treatment is better adjusted. Hypothyroidism in previously hyperthyroid cats can also occur following surgical removal or other means of destruction of the thyroid glands.

Click here to read more on canine hypothyroidism.

Q: How common is feline hyperthyroidism? What causes it?

A: Hyperthyroidism is quite common in kitties, occurring in roughly 10% of middle-aged or elderly cats over 10 years old (most average 12-13 years of age or older). Cats may be slightly older or younger than this average when diagnosed, but hyperthyroidism is exceedingly uncommon in felines younger than 6 years. Hyperthyroidism occurs due to adenomatous hyperplasia of the thyroid gland, meaning excessive growth of the hormone-releasing tissue of the thyroid gland, resulting in an enlarged thyroid gland and an increased level of thyroid hormones. This is a benign tissue growth, meaning it is not cancer and will not spread to other organs. The cause of this thyroid gland hyperplasia is not fully understood. The theoretical trigger is an accumulation of endocrine disrupters known as goitrogens, chemical molecules toxic to the thyroid gland, such as polybrominated diphenyl ethers found in fire retardants and bisphenol-A (BPA) in some plastics.

On occasion, thyroid tissue can start growing in areas outside the thyroid gland, such as in the mediastinum, a space inside the chest cavity. This type of tissue is known as ectopic tissue and produces thyroid hormones the same as the thyroid gland. This ectopic thyroid tissue can also contribute to hyperthyroidism. Very rarely, carcinoma of the thyroid gland, a malignant cancer, can secondarily induce hyperthyroidism.

Q: What are the clinical signs of hyperthyroidism?

A: As previously mentioned, you may notice increased appetite and water consumption, increased litterbox usage, hyperactivity and more meowing behavior, and possible GI upset in your hyperthyroid kitty. If you observe these signs as well as any of the following described below, it’s time to schedule a veterinary appointment. Your vet may also notice other abnormalities, as listed below, during your cat’s physical exam.

• Changes that you may observe in your hyperthyroid cat at home:
  • Increased appetite (known as polyphagia) due to an increased metabolic rate that “burns” through energy more quickly, requiring your kitty to “refuel” more often
    • Though less common, some hyperthyroid cats may actually have a decreased appetite.
  • Weight loss (despite a good appetite)
  • Vomiting and/or diarrhea (most commonly an increased amount of softer stool rather than watery diarrhea)
  • Increased activity, less sleep, restlessness
  • Increased vocalization and meowing
  • Weakness
  • Increased urination and water consumption (known as polyuria and polydipsia, respectively; commonly abbreviated PU/PD)
  • Poor hair coat quality; sometimes oily skin (or seborrhea)
  • Sudden blindness due to bleeding in the retina secondary to high blood pressure
  • Panting
    • However, if your cat is open-mouth breathing or gasping for air, seek emergency veterinary attention immediately as this can be a sign of life-threatening heart disease or an asthma attack.

   

• Additional abnormalities that your veterinarian may detect upon physical examination:
  • A thin body condition
  • An enlarged thyroid gland — a.k.a. a “goiter” or “goitre” (the thyroid gland is not palpable, or able to be felt, in normal cats)
  • Cardiovascular abnormalities: fast heart rate, heart murmur, high blood pressure, or occasionally irregular heart rhythm (read on to learn more about how hyperthyroidism affects the heart)
  • Eye examination changes or neurologic abnormalities: As mentioned, hyperthyroidism can cause high blood pressure (a.k.a. hypertension). This increased pressure inside blood vessels can cause them to leak or burst, resulting in bleeding. If bleeding occurs in the retina (at the back of the eye) or if high blood pressure induces a retinal detachment, sudden vision loss can occur. Bleeding inside the brain can result in an abnormal neurologic examination. Your vet will want to measure your cat’s systolic blood pressure and initiate prompt treatment if your pet is hypertensive to minimize the risk of further damage to the retinas, central nervous system (CNS), as well as the kidneys (more on that later).

Q: How can hyperthyroidism affect the heart and cardiovascular system? Why does it need to be diagnosed and treated?

A: With an increased metabolic rate due to an excess level of thyroid hormones in a hyperthyroid state, the body’s organs require more oxygen to keep up with this higher rate of chemical reactions. Therefore, the heart must pump faster and harder to send out oxygen-rich blood to the rest of the body. (This includes increased blood flow to the kidneys. Why am I mentioning this? Act like a cat and let curiosity get the better of you and keep reading below.) In time, hyperthyroidism can cause left ventricular hypertrophy, increased thickness of the muscular wall of the left ventricle — the main blood-pumping chamber of the heart. If left untreated, hyperthyroidism can lead to secondary heart failure. As listed above, a hyperthyroid cat may have a fast heart rate (tachycardia) or heart murmur (abnormal heartbeat sounds due to irregular blood flow through the heart). Hyperthyroid cats also commonly have high blood pressure (hypertension). Though less common, an irregular heart rhythm (arrhythmia) may also coincide with these other abnormalities, though this occurs more often if your cat has pre-existing heart disease coinciding with hyperthyroidism (and not necessarily caused by the thyroid disease itself).

Based on his or her findings, your vet may also wish to take chest X-rays (radiographs) to better visualize heart size or an electrocardiogram (ECG or EKG) if your cat has a cardiac arrhythmia. Depending on the degree of your cat’s heart disease, your vet may recommend referral to a veterinary cardiology specialist for a complete cardiac assessment, including an echocardiogram (echo), which is an ultrasound (sonogram) of the heart.

If significant tachycardia and cardiac arrhythmias persist despite the start of treatment of hyperthyroidism or if your cat does not tolerate certain treatments well, your vet may recommend cardiac medications known as beta-blockers in order to control your cat’s cardiac side effects.

Click here to read our article on the basics of feline and canine heart disease. ❤️

Q: How is hyperthyroidism diagnosed?

A: Paired with the physical exam findings and description of your cat’s at-home behavior, your vet will likely already be suspicious of hyperthyroidism (and so will you after reading this!). For confirmation purposes as well as a way of ruling out coinciding illnesses, your vet will run routine bloodwork (CBC and chemistry) in addition to a thyroid hormone panel. This specialized blood test, which measures thyroid hormone levels (total T4 and free T4, which will be elevated in hyperthyroidism), can be performed in-house in some veterinary hospitals while it may need to be sent to a special laboratory for other clinics. The severity of your cat’s hyperthyroidism is related to the degree of elevation of the thyroid hormones above the normal reference range.

Other common bloodwork changes occurring with hyperthyroidism include an increased number of red blood cells or PCV (termed erythrocytosis), elevated liver enzymes (including ALP and ALT), an increased phosphorus level (due to increased bone metabolism), and sometimes a slight increase in blood sugar (or hyperglycemia). Your cat’s bloodwork may also reveal elevations in BUN and creatinine; this condition is known as azotemia. Azotemia can indicate dehydration and kidney dysfunction. While hyperthyroidism does not always cause kidney dysfunction (though it can due to hypertension), chronic kidney disease (CKD) is a common concurrent finding in older cats — those right around the same age in which we also diagnose hyperthyroidism; thus, these two diseases may be diagnosed at the same time. Knowing the level of your cat’s azotemia in the face of hyperthyroidism is essential as treatment for the thyroid disorder can actual worsen kidney function if pre-existing disease exists. (How, you may ask? Read on to understand why frequent rechecks are vitally important in a hyperthyroid cat.) Your vet may also collect urine for a urinalysis to further analyze urinary tract health.

Occasionally, the initial thyroid panel may appear within normal values in a hyperthyroid cat. If your vet is still suspicious of hyperthyroidism, he or she may either repeat this thyroid panel at a later time or perform specialized blood tests, such as TSH (thyroid-stimulating hormone) or T3 suppression, or else suggest referral to a specialized radiology center for radionuclide imaging (a type of special imaging known as nuclear scintigraphy).

For more information on interpreting your pet’s bloodwork, click here. Curious about how to read your pet’s urinalysis? Click here. And learn more about Chronic Kidney Disease (CKD) here.

Q: What are the treatment options for hyperthyroidism?

A: Four main treatment options currently exist for the treatment of hyperthyroidism: methimazole (an anti-thyroid drug), a special diet (Hill’s y/d), surgical removal of the thyroid gland (thyroidectomy), and radioactive iodine treatment. The majority do not achieve a true cure but merely manage the disease by attempting to achieve a normal thyroid hormone level (i.e. a euthyroid state). Each option has its advantages and disadvantages, ranging from potential side effects and expense.

• Methimazole: Methimazole (and the related drug carbimazole as used in Europe and Australia) are “anti-thyroid” medications that inhibit the formation of thyroid hormones. Methimazole is the most common treatment method and is available in both oral and transdermal formulations. The “transdermal” option is a topical gel that is applied to the skin of the patient’s ears where it is absorbed into the bloodstream. (While easier to administer than a pill to most cats, this gel can cause itching and open sores on the ears secondary to scratching.) Methimazole usage takes up to 3 – 4 weeks before improvement is noted, and the dosage may need to be adjusted to achieve desired thyroid hormone levels. Once this relatively inexpensive medication is started, bloodwork needs to be performed every 2 – 4 weeks (typically for 2 – 3 months) to closely monitor for normalizing thyroid hormone levels as well as to detect potential side effects (which include vomiting and decreased appetite, possible severe blood cell abnormalities, liver toxicity, and facial itching). Due to these potential side effects, some cats are unable to tolerate methimazole and require another treatment option.
• Special diet: Iodine is a mineral obtained from our diet, and it is necessary to produce thyroid hormones. By reducing the content of dietary iodine, the production of thyroid hormones should decrease as well. Currently, Hill’s® Prescription Diet® y/d® Feline Thyroid Care is the only low-iodine veterinary cat food for the management of hyperthyroidism. It comes in both dry and canned food varieties. This food is only effective at managing hyperthyroidism if it is the sole food source that your cat consumes — this means no other brands of cat food, no human foods, and no treats. Usage of this special diet also prevents your cat from eating other potential therapeutic prescription diets if he or she needs them in the future. Hill’s® y/d® takes a bit longer than methimazole to lower thyroid hormone levels, but it is shown to be safe and effective at managing hyperthyroidism in the majority of cats (though not all), including those who cannot tolerate methimazole. A thyroid panel should be performed every 4 – 8 weeks after starting this prescription food until thyroid hormone levels have normalized.
• Surgery: Surgical removal of both thyroid glands (termed thyroidectomy) is effective at curing hyperthyroidism in many cases, yet surgery carries the risk of several significant complications. Anesthetic risks are of potential issue as well as permanent post-surgical concerns, including hypoparathyroidism resulting in dangerously low calcium levels (caused by accidental surgical removal of the nearby calcium-regulating parathyroid glands that lie in close proximity to the thyroid glands), Horner’s syndrome (nerve abnormalities of the face and eyes), laryngeal paralysis (abnormal activity of the larynx, resulting in possible difficulty breathing and aspiration pneumonia), and hypothyroidism (low thyroid hormone levels). Additionally, a thyroidectomy does not remove potential ectopic thyroid tissue that may be actively producing thyroid hormones (as opposed to our next treatment option…).
• Radioactive iodine treatment: The ideal treatment of choice for feline hyperthyroidism is radioactive iodine treatment (also called radioiodine therapy). This treatment involves the administration of iodine-131 (I-131), a radioactive form of iodine that travels to the thyroid gland (and any ectopic thyroid tissue), destroying overactive cells. Since iodine is used nearly solely by the thyroid gland, other organs of the body are unaffected. Radioactive iodine achieves a cure in 95% of hyperthyroid cats after a single non-invasive treatment, though some cats may require additional treatments to reach success. It is a relatively safe option, carrying minimal risks (though a small subset of cats can develop hypothyroidism as a result, yet most of these cases are only temporary). Though it is the preferred treatment of choice, I-131 is not readily available in most areas, and is a referral treatment at university or specialty veterinary hospitals. Additionally, this treatment is quite expensive and requires that your cat be in an isolation ward at the veterinary treatment facility for several days to prevent anyone from coming into contact with their radioactive waste products (urine, feces, etc.). Prior to radioiodine treatment, cats should ideally have a trial taking methimazole to observe for any ill effects to the kidneys (meaning, the development of azotemia). For more information on the benefits of radioactive iodine therapy, visit Virginia-Maryland College of Veterinary Medicine‘s website and listen to audio on I-131 treatment.

Q: Why does my vet recommend such frequent rechecks?

A: Because hyperthyroidism is so common in older cats, a thyroid hormone blood screening should ideally be part of all routine tests for cats 7 years of age and older. As discussed, thyroid hormone regulation can take awhile to occur after either methimazole or Hill’s® y/d® prescription food is started, requiring regular recheck bloodwork to track thyroid hormone levels to ensure that they neither remain too high (increasing your cat’s odds of developing cardiovascular disease) or drop too low (i.e. into the hypothyroid range). Additionally, due to the potential side effects of methimazole, bloodwork needs to be closely monitored. Liver toxicity is detected as high liver enzyme values on bloodwork. Additionally, the development of severe blood cell abnormalities may alert your vet to stop methimazole and suggest another treatment option for your hyperthyroid cat. Such blood cell changes include low white blood cells (leukopenia), increasing your cat’s risk of infection; low platelet count (thrombocytopenia), making your cat more susceptible to bleeding; and red blood cell destruction (hemolytic anemia), which can be life-threatening.

Furthermore, your cat’s blood pressure should be regularly monitored since hyperthyroidism can cause hypertension, as previously mentioned. However, even if your cat’s blood pressure is normal at the time of its diagnosis of hyperthyroidism, some cats can develop hypertension up to 6 months later following treatment.

One of (if not THE) most important reasons that your vet will recommend what seem like such frequent rechecks is to track your cat’s kidney function. As explained, hyperthyroidism increases metabolic rate, thereby leading to a higher demand of oxygen and nutrients by the body. This demand is met by an increased heart rate and heart pumping force to pass more nutrient-rich blood to the rest of the body. This means that the kidneys receive more blood flow, too, which speeds up glomerular filtration rate (GFR) — the kidneys’ rate of filtering out toxins and waste products from the bloodstream (such as BUN and creatinine). However, up to 14 – 40% of cats with hyperthyroidism have concurrent chronic kidney disease (CKD). Because the increased blood flow aids the kidneys in flushing out toxins from the body in a hyperthyroid state (i.e. before treatment of hyperthyroidism has been initiated, when thyroid hormone levels are still high), CKD may not yet be detectable on urinalysis or bloodwork (where it normally shows up as an increased BUN and creatinine level, or azotemia). However, once hyperthyroidism is treated and thyroid hormone levels reduce to a normal level, metabolic rate and thus blood flow to the kidneys decrease. Since the kidneys do not receive as much blood as they once did, pre-existing renal dysfunction can be “unmasked” in the form of the detection of azotemia on bloodwork. If CKD is “unmasked” or develops following the start of hyperthyroidism treatment, the management of these two diseases must be balanced — a difficult feat. For this reason, most veterinarians require a methimazole trial prior to radioactive iodine therapy to determine if your cat has pre-existing CKD and to assess to which degree that kidney dysfunction and azotemia will be at a normal or euthyroid state prior to a permanent treatment such as radioiodine. (Please note that previous recommendations of maintaining a cat with CKD at mildly hyperthyroid levels is no longer indicated since the development of hypertension secondary to hyperthyroidism can add to the progression of CKD with time.)

Hopefully, this Q & A session has answered the majority of your basic inquiries about feline hyperthyroidism — the “cat’s meow” of endocrine diseases. For an individualized diagnostic and treatment plan, your primary veterinarian will be able to guide you on the best course of action for your kitty should he or she be diagnosed with hyperthyroidism.

Still have some questions on this metabolic disorder? Please contact us and follow us on Facebook to have your dog and cat health or safety questions addressed.

– Maranda Elswick, DVM