Hyperthyroidism and your cat

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Hyperthyroidism and your cat

Is your cat ravenously hungry, losing weight and acting restless or aggressive? He could have hyperthyroidism, a common feline health problem.

Most cat caregivers know that a change in appetite and weight is cause for concern. In most cases, suppressed appetite leads to weight loss, while a large appetite results in weight gain or obesity. But what if your cat has become ravenously hungry – and is actually losing weight? These symptoms can be caused by a number of health problems, but hyperthyroidism is one of most common, especially if your kitty is getting older and is displaying addition signs such as vomiting, hyperactivity, vocalization and aggression. In some countries, in fact, hyperthyroidism is seen in at least 10% of older cats. Let’s look at some of the causes of this condition, along with how it’s diagnosed and managed.

Manmade chemicals are culprits

Cats are exposed to many of the same toxins humans are, since they often share the same indoor environments. Even though cats are much smaller than we are, don’t eat the same diets, and have different activity habits, they react to toxins parallel with humans, especially children, when it comes to manmade chemicals.

Two chemicals that have been associated with feline hyperthyroidism are per- and polyfluoroalkyl substances (PFAS), and flame retardant chemicals such as polybrominated diphenyl ethers (PBDEs).

1. PFAS

Per- and polyfluoroalkyl substances (PFAS) have been used in a variety of industries in the US and around the world since the 1940s. They persist in the environment, in drinking water and foods, and in the bodies of humans, fish, and other animals. Certain PFAS are no longer permitted to be manufactured in the US, are but still produced internationally and can thus be imported in consumer goods such as carpets, leather and apparel, textiles, paper and packaging, coatings, rubber and plastics.

When studied, serum from hyperthyroid cats showed significantly higher PFAS residues when compared to non-hyperthyroid cats. In humans, exposure to PFAS is associated with low infant birth weights, effects on the immune system, cancer, and disruption of thyroid hormone.

2. PBDEs

Polybrominated diphenyl ethers (PBDEs) are the most abundantly-used flame retardants. They are used in many consumer goods, including electronics, furniture, building materials, and automobiles, to slow or prevent the start or growth of fire. These flame retardants have been shown to have many effects on the body, including disruption of the endocrine system, which also occurs with PFAS. PBDEs are easily released from products and get into air and dust; they can also enter the environment through manufacturing, wear and tear of products during use, and product disposal.

One study showed that household cats have much higher serum levels of flame retardants in their bodies than humans.

Potential exposure to flame retardants is higher in children due to their typical hand-to-mouth behavior and closeness to the floor — PBDE levels are known to be higher in children than in adults. Cats, of course, are also closer to the floor than we are. PBDEs can also accumulate in household dust that ends up on animal fur. The animals, especially cats, ingest the chemicals when they lick themselves during grooming.

One study showed that household cats have much higher serum levels of flame retardants in their bodies than humans; interestingly, dogs have lower levels because they are metabolically better equipped to degrade these compounds.

Do your best to reduce your cat’s exposure (and your own!) to these chemicals. Admittedly, this isn’t easy to do in today’s world, but some helpful ideas are to use more natural materials for floor coverings and furnishings, buy domestically-produced clothing and household items whenever you can, and limit the use of plastics and other synthetic materials in the home.

Screening allows your vet to detect mild or early hyperthyroidism before the disorder progresses to more severe disease.

Diagnosing hyperthyroidism

Clinical hyperthyroidism

Veterinarians are encouraged to start screening all cats for hyperthyroidism when the animals are eight to ten years of age. Screening allows us to detect mild or early hyperthyroidism before the disorder progresses to more severe disease.

If the total T4 (thyroxine) level is in the upper third of the lab reference range, the cat could be suffering from mild hyperthyroidism. Fluctuations in the circulating levels of T4 and T3 (triiodothyronine) is common in cats with mild hyperthyroidism, which can make diagnosis more difficult. If a thyroid nodule cannot be palpated in the neck, this could rule out the diagnosis of hyperthyroidism – however, tiny thyroid nodules can be very difficult to palpate.

The usual diagnostic tests include total and free T4 testing plus cTSH (canine thyroid stimulating hormone, since no feline-specific assay is available). Additional tests include T3 suppression test and thyroid scintigraphy, if needed, to establish the diagnosis.

Borderline or occult hyperthyroidism

Cats with mild or occult hyperthyroidism almost always have a low cTSH value, at or below the detection limit of the assay (<0.03 ng/ml). The combined measurements of the total T4 and free T4 with cTSH are helpful in the diagnosis of hyperthyroidism, especially in cats with chronic kidney disease. The frequency of chronic renal failure in older cats often suppresses the total T4 level non-specifically, which can mask the diagnosis. Further, cats testing with a low cTSH are likely to have histological evidence of nodular thyroid disease, and thus should be considered sub-clinically hyperthyroid.

If the cat shows apparent clinical signs of hyperthyroidism, the best diagnostic approach is to closely monitor his body weight, heart rate, and thyroid size every few months.

While feline hyperthyroidism can be an alarming diagnosis, it can be treated and managed (see sidebar above). Keep an eye on your kitty, especially if he’s getting a bit older. If he’s suddenly eating everything in sight, yet losing weight, have him checked out by your veterinarian.