Hyperthyroidism in Cats: Signs, Treatment & Kidney Risks

Your senior cat has been eating more than usual but losing weight anyway. Maybe drinking more water, maybe a little more restless at night. It’s easy to write off as aging, but this combination of signs is one of the most recognizable patterns in veterinary medicine. It’s often hyperthyroidism, and it’s the most common endocrine disorder in cats.

What hyperthyroidism is

The thyroid gland sits in the neck and produces hormones (T4 and T3) that regulate metabolism. In hyperthyroidism, the gland becomes enlarged and overactive, flooding the body with thyroid hormone. The metabolic rate goes up, the heart works harder, and the cat burns through calories faster than it can eat them.

In about 97-99% of cases, the cause is a benign overgrowth of thyroid tissue, either adenomatous hyperplasia or adenoma. Thyroid carcinoma (malignant tumor) accounts for only 1-3% (Peterson & Broome, 2015). It’s a disease of older cats. The typical age at diagnosis is around 12-13 years, and it rarely shows up in cats younger than 8.

How common is it? Studies consistently find a prevalence of roughly 10-12% in cats over 9 years old (Peterson, 2012; Edinboro et al., 2004). That’s about 1 in 10 senior cats. Siamese and Himalayan breeds appear to have a somewhat lower risk, though the reason isn’t understood.

The disease was first described in 1979 and has been increasing since. Whether the rise reflects genuinely more cases, better detection through routine screening, or longer lifespans giving cats more time to develop it is still debated (Peterson, 2012).

Signs to watch for

The hallmark is weight loss despite a normal or increased appetite. This combination is reported in 70-90% of hyperthyroid cats at diagnosis and is unusual enough that it should prompt a vet visit in any older cat.

Other common signs include:

• Increased thirst and urination
• Vomiting or diarrhea
• Restlessness or irritability
• Fast heart rate
• Unkempt, greasy, or matted coat

Your vet may also detect a heart murmur, a gallop rhythm, or a palpable thyroid nodule during a physical exam. A senior cat who gets newly noisy or hyperactive at night — especially combined with weight loss — is a classic hyperthyroid pattern; the cat sleep guide covers other sleep changes that signal illness.

There’s an atypical form called “apathetic hyperthyroidism” that shows up in about 5-10% of cases (Carney et al., 2016). Instead of the expected hyperactivity, these cats become lethargic, depressed, and lose their appetite. This presentation can delay diagnosis because it doesn’t match what people expect.

If your cat is over 8, losing weight, and eating the same or more, that’s worth checking. If the weight loss comes with increased thirst or vomiting, even more so.

How it’s diagnosed

The standard first step is a blood test measuring total T4 (TT4). In most hyperthyroid cats, TT4 is clearly elevated. The test has a sensitivity of about 91-98% for moderate-to-severe disease (Peterson, 2013).

There’s a diagnostic gray area: cats with early or mild disease may have a TT4 that’s high-normal rather than obviously elevated. Concurrent illness can also suppress T4 into the normal range even when hyperthyroidism is present (euthyroid sick syndrome). If suspicion remains despite a normal TT4, your vet may recommend retesting in a few weeks, measuring free T4 by equilibrium dialysis, or checking TSH levels (which need to be interpreted alongside T4, not on their own).

Thyroid scintigraphy (a nuclear scan using technetium-99m) is the gold standard for locating exactly where the overactive thyroid tissue is, including ectopic tissue. It’s most relevant when surgery or radioactive iodine treatment is being planned.

Your vet will also likely run a full blood panel. Elevated liver enzymes (ALT and ALP) are found in roughly 60-75% of hyperthyroid cats, which usually resolve after treatment (Carney et al., 2016). Kidney values (creatinine, BUN, SDMA) matter a great deal for treatment planning, as explained in the section below. A blood pressure check and cardiac evaluation may also be recommended, since untreated hyperthyroidism can cause secondary thickening of the heart muscle that often improves once thyroid hormone normalizes.

Treatment options

There are four main approaches. None is universally best. The right choice depends on the individual cat’s health, the owner’s circumstances, and what’s available locally. All treatment decisions should be made with your veterinarian.

Radioactive iodine (I-131)

This is considered the gold standard for definitive treatment by most veterinary endocrinologists. A single dose of radioactive iodine is given (orally or by injection), and it selectively destroys the overactive thyroid tissue while sparing normal tissue. The cure rate is about 95% with a single treatment (Peterson & Becker, 1995).

The main drawbacks are practical. The cat needs to be hospitalized in a licensed radiation facility for several days (requirements vary by country; in some regions it can be 1-3 weeks). Not all areas have facilities that offer it, and the cost is significant. About 2-5% of cats become permanently hypothyroid afterward and may need supplementation.

Because I-131 is irreversible, most veterinarians recommend a methimazole trial first to check how renal function responds before committing to permanent treatment.

Anti-thyroid medication

Methimazole (known as thiamazole in some countries, or its prodrug carbimazole) is the most common initial treatment worldwide. It works by blocking thyroid hormone production. It does not destroy the thyroid tissue, so treatment is ongoing.

The drug achieves euthyroidism (normal thyroid levels) in over 90% of cats within 2-4 weeks at the dose determined by the veterinarian. It comes in tablet, liquid, and transdermal forms (applied to the inside of the ear). If using the transdermal form, always wear gloves or finger cots when applying it — the drug can be absorbed through human skin, which is particularly dangerous for pregnant women. The key advantage of methimazole is reversibility: if the cat’s kidney function worsens after thyroid levels normalize, the dose can be adjusted or stopped.

Side effects are usually mild and often resolve on their own: GI upset (10-20%), lethargy, or facial itching (2-3%) (Trepanier, 2007). Rare but serious side effects include liver toxicity and bone marrow suppression, which is why regular blood work is necessary. The AAFP guidelines recommend rechecking T4, CBC, and kidney/liver values at 2-4 weeks, then every 3-6 months (Carney et al., 2016).

Surgery

Surgical removal of the thyroid gland (thyroidectomy) can be curative, but it carries specific risks. The parathyroid glands, which regulate calcium, sit right next to the thyroid. Damage during surgery can cause dangerous drops in blood calcium (hypocalcemia), reported in 5-25% of bilateral procedures. The cat also needs anesthesia, which carries added risk in older patients with cardiac involvement. Stabilization with methimazole before surgery is standard practice.

Iodine-restricted diet

A prescription diet that severely limits dietary iodine (the raw material for thyroid hormone) can reduce T4 levels. It must be the only food the cat eats: no treats, no table scraps, no other cat’s food. Even a few licks from another cat’s bowl or crumbs of regular food off the floor can undermine the entire approach.

This approach is more controversial than the others. While studies show it can normalize T4 in many cats, the underlying thyroid pathology continues to grow because it’s not being treated. Compliance is extremely difficult, especially in multi-cat households. The AAFP guidelines list it as an option primarily when other treatments aren’t feasible (Carney et al., 2016). If your vet suggests this route, discuss the limitations honestly.

Why hyperthyroidism and kidney disease are linked

If there’s one thing to take away from this article, it’s this.

Excess thyroid hormone increases blood flow to the kidneys, artificially inflating their filtration rate (GFR). In a cat that already has early kidney disease, this masks the problem. Blood tests for kidney function come back looking better than the kidneys actually are.

When hyperthyroidism is treated and the thyroid hormone level drops to normal, blood flow to the kidneys decreases to its true level. Kidney values that were hidden behind the hyperthyroid state now become visible. This is not the treatment causing kidney damage. The kidney disease was already there; it was just being concealed.

Studies have found that 15-40% of hyperthyroid cats develop measurable kidney dysfunction within the first few months of achieving normal thyroid levels (DiBartola et al., 1996; Williams et al., 2010). One study also showed that cats who become hypothyroid from over-treatment fare even worse, with shorter survival times and faster kidney decline (Williams et al., 2010).

This is exactly why most veterinarians use a methimazole trial before considering irreversible treatments like I-131 or surgery. The trial period lets everyone see what happens to the kidneys when the thyroid is controlled. If significant kidney disease emerges, the treatment plan can be adjusted — balancing thyroid control against kidney preservation. For more on kidney disease itself, our kidney disease guide covers staging, treatment, and what to expect.

What you can do at home

You can’t diagnose or treat hyperthyroidism at home, but you can catch it early.

Track your cat’s weight. Of everything on this list, this one matters most for early detection, and not just for hyperthyroidism. A cat that’s gradually losing weight over months may not look different day to day, but the trend on a scale tells the real story. Weigh your cat every 2-4 weeks and note the numbers.

Watch for the combination. Weight loss plus increased appetite. Weight loss plus increased thirst. These pairings in an older cat should prompt a vet visit.

Keep up with senior screening. The AAFP recommends annual wellness bloodwork including T4 for all cats aged 7 and older, and every 6 months for cats over 10. Early detection means more treatment options and better outcomes.

If your cat is already on treatment for hyperthyroidism, follow your vet’s monitoring schedule closely. Skipping rechecks is risky because both the thyroid levels and kidney function need ongoing surveillance.