# Overactive Thyroid in Cats: Signs and Management An overactive thyroid in cats, called hyperthyroidism, is a condition in which the thyroid glands produce too much thyroid hormone. The classic picture is an older cat who loses weight even though she eats more than ever, drinks a lot, urinates a lot, and may vomit or act restless. The diagnosis is confirmed with a blood test for total thyroxine (T4), supported by a complete blood count, biochemistry panel, urinalysis, and a blood pressure check. Treatment options are a prescription iodine-restricted diet, daily methimazole (as a pill or a transdermal gel), surgical removal of the thyroid gland, or radioiodine therapy. Each has trade-offs, and all of them require monitoring because controlling the thyroid can reveal hidden kidney disease. This article is educational and is not a substitute for veterinary diagnosis or treatment. ## Owner Triage Summary If your older cat is losing weight but eating well, that combination deserves a veterinary visit within days, not weeks. If your cat is also breathing fast, collapsing, or has a racing heart that you can feel through the chest wall, treat it as urgent. Call your veterinarian promptly if you notice any of these: - Weight loss with a normal or increased appetite - Increased drinking and urination - Vomiting or diarrhea that keeps coming back - Restlessness, nighttime yowling, or unusual aggression - A dull, greasy, or matted coat that will not groom clean - Fast heart rate, a heart murmur, or labored breathing - Muscle weakness, a wobbly back leg gait, or episodes of unusual limb movements ## What the Thyroid Gland Does The thyroid gland sits in the neck, wrapped around the windpipe. It has two lobes, one on each side. The gland takes iodine from the diet and uses it to build thyroid hormones, mainly thyroxine (T4) and triiodothyronine (T3). These hormones set the body's metabolic speed. In a healthy cat, the pituitary gland in the brain releases thyroid-stimulating hormone (TSH), which tells the thyroid how much hormone to make. When thyroid hormone levels are right, TSH drops. That feedback loop keeps metabolism steady. In [feline hyperthyroidism](/knowledge/veterinary-medicine/clinical-methods/feline-hyperthyroidism-diagnosis-management), this loop breaks. The thyroid gland develops one or more nodules that produce hormone on their own, without waiting for TSH. These nodules are described as autonomously functioning, meaning they ignore the normal control signal [1]. Because the pituitary can no longer restrain them, thyroid hormone pours into the bloodstream and metabolism runs too fast. Feline hyperthyroidism is clinically and histologically similar to toxic nodular goiter in people, with single or multiple overactive nodules in the gland [1]. It is not the same disease as Graves disease, which is driven by circulating antibodies. In cats, the overgrowth and overproduction come from the nodules themselves. ## How Common Is It, and Who Gets It? Hyperthyroidism is one of the most common disorders of middle-aged and senior cats [1]. In a large laboratory survey from Spain that measured total T4 in 27,888 cats, 1,722 cats (6.2 percent) were classified as hyperthyroid [2]. Within that hyperthyroid group, ages ranged from 2 to 25 years, with a median age of 14 years, and 88.8 percent of the cats with available age data were seniors over 10 years old [2]. The same survey graded cases as mild, moderate, or severe based on how high the T4 was, which matters because mildly affected cats can be easy to miss on a single test. Age is the strongest risk factor. The disease is uncommon in young cats and becomes progressively more likely as a cat moves into the senior years. Cats older than 6 years with any signs of heart disease should be screened for hyperthyroidism, because the two problems frequently travel together [3]. No single breed predisposition is strong enough to change how you screen a cat. The practical takeaway is age-based: any cat in the double-digit years with unexplained weight loss deserves a T4 test. ## Why the Signs Look the Way They Do Thyroid hormone speeds up nearly every organ system. That explains why the signs of an overactive thyroid in cats are so widespread. - **Weight loss despite a good appetite.** The body burns calories faster than the cat can take them in, so fat and muscle are consumed even while the cat eats ravenously. Weight loss with a normal or increased appetite is the hallmark sign [1]. - **Increased thirst and urination.** Faster metabolism increases fluid turnover, and the kidneys filter more. - **Vomiting and diarrhea.** A sped-up gut moves food through quickly and tolerates it poorly. - **Hyperactivity, restlessness, and vocalization.** The nervous system is overstimulated. Some cats become irritable or aggressive. - **Unkempt coat.** Cats with hyperthyroidism often stop grooming well, and the coat becomes greasy, matted, or dull. Weight loss also makes the coat look poor. - **Fast heart rate and heart murmur.** Thyroid hormone drives the heart to beat faster and harder. Many hyperthyroid cats have tachycardia, and a systolic murmur is common. Over time the heart muscle thickens and the heart can enlarge [3]. - **Muscle weakness and wasting.** Cats may lose muscle along the spine and develop a stiff, wobbly walk. A less familiar sign is a movement disorder. In a series of seven cats, paroxysmal dyskinesia, which looks like sudden involuntary limb movements, occurred alongside hyperthyroidism, and the episodes stopped completely once thyroid function was restored with treatment [4]. Hyperthyroidism should be on the differential list for a cat with these episodes, even when other classic signs are absent [4]. ## The Heart Connection Cardiac changes are common in hyperthyroid cats and are often reversible. Hyperthyroid cats frequently develop cardiomegaly, with thickening of the left ventricle and dilation of the left atrium [3]. These changes usually resolve once thyroid hormone levels are normalized, whether by radioiodine, thyroidectomy, medical therapy, or an iodine-restricted diet [3]. That reversibility has a limit. If a cat already has a primary heart disease such as hypertrophic cardiomyopathy, the heart changes may not fully reverse, because the underlying disease remains [3]. This is why cats with hyperthyroidism should have echocardiographic evaluations before and after treatment, and why any cat over 6 years old with heart signs should be screened for thyroid disease [3]. Uncontrolled hyperthyroidism is not harmless to the heart. In one case report, a 16-year-old cat with chronic, poorly controlled hyperthyroidism developed congestive heart failure attributed to myocardial injury from the untreated disease [5]. The cat had stopped regular follow-up and had been off appropriate treatment. This is the scenario that good monitoring is designed to prevent. ## Causes and What Else It Could Be The basic lesion in feline hyperthyroidism is an excessive intrinsic growth capacity of some thyroid cells, producing nodules that secrete hormone independently of TSH [1]. Iodine deficiency, nutritional goitrogens, and environmental disruptors may contribute to the disease, though the exact triggers are not settled [1]. Several other conditions can mimic parts of the picture, which is why testing matters: - **Chronic kidney disease.** Causes weight loss, increased drinking, and increased urination, but appetite is usually reduced rather than increased. - **Diabetes mellitus.** Causes weight loss, increased appetite, increased drinking, and increased urination. Cats can have both diabetes and hyperthyroidism. - **Gastrointestinal disease, including lymphoma.** Causes weight loss, vomiting, and diarrhea, often with a poor appetite. - **Primary heart disease.** Causes murmur, tachycardia, and sometimes weight loss, without high thyroid hormone. - **Chronic stress or behavioral change.** Causes vocalization and restlessness, without the metabolic signs. Because these overlap so heavily, a cat with weight loss and a good appetite should be tested for hyperthyroidism rather than treated symptom by symptom. ## Symptom Checklist for Owners Use this checklist to organize what you have noticed before the appointment. Bring it with you. **Appetite and weight** - Eating more than usual, or constantly begging - Losing weight or feeling bony along the spine - Losing muscle over the back and shoulders **Water and litter box** - Drinking more, refilling the bowl more often - Larger or more frequent urine clumps - Accidents outside the box **Digestion** - Vomiting, especially after eating - Diarrhea or soft stool - Eating non-food items **Behavior and activity** - Restlessness, pacing, or nighttime yowling - Irritability or new aggression - Unusual limb movements or episodes of stiffness **Coat and body** - Greasy, matted, or dull coat - Poor grooming - Fast heartbeat or a murmur heard by your vet - Fast breathing at rest ## How Hyperthyroidism Is Diagnosed ### Step 1: Physical Examination Your veterinarian will weigh your cat, feel the neck for enlarged thyroid lobes, listen to the heart for rate and murmur, check blood pressure, and assess body and muscle condition. A palpable thyroid nodule plus weight loss in an older cat strongly suggests the diagnosis [1]. ### Step 2: Total T4 The total thyroxine (TT4) blood test is the primary diagnostic test. A high TT4 confirms hyperthyroidism [1]. In the large survey, a TT4 above 60.4 nmol/L (4.7 µg/dL) was used as the cutoff for classifying a cat as hyperthyroid [2]. One normal T4 does not fully rule the disease out. Early or mild cases can fluctuate around the top of the reference range. If your cat has classic signs and a T4 in the upper part of the normal range, your veterinarian may repeat the test or add further testing. ### Step 3: Free T4 When the Total T4 Is Borderline When the total T4 is borderline or normal but suspicion remains high, measuring free T4 helps. Free T4 is the unbound, active fraction of the hormone, and it can be elevated in hyperthyroid cats whose total T4 still looks acceptable. This is the standard next step for a borderline case. ### Step 4: Complete Blood Count, Biochemistry, and Urinalysis These tests do not diagnose hyperthyroidism on their own, but they are essential for safe treatment. They assess kidney function, liver enzymes, blood sugar, and [red blood cell](/blog/guides/red-blood-cell) parameters. Hyperthyroid cats often have high liver enzyme activity, and red blood cell counts tend to run high because thyroid hormone stimulates the bone marrow. Red cell measures track thyroid status during treatment. In a prospective study of 43 hyperthyroid cats treated with methimazole, hematocrit and mean corpuscular volume tracked T4 closely within individual cats, and a rising hematocrit or MCV during treatment can signal that control is slipping and prompt a T4 recheck [6]. That study does not replace hormonal monitoring, but it shows how much the blood count moves with thyroid status [6]. ### Step 5: Blood Pressure Hypertension is a recognized concern in hyperthyroid cats, and blood pressure should be checked as part of the workup. High blood pressure can damage the kidneys, eyes, heart, and brain, and it changes how aggressively you treat. ### Step 6: Kidney Values Before and After Treatment This step is central. Hyperthyroidism increases blood flow to the kidneys and can artificially flatter kidney values. When you bring thyroid hormone down, kidney function can appear to worsen. In a controlled trial of 34 cats, serum creatinine increased significantly after 90 days only in the methimazole-treated groups, not in the iodine-restricted diet group [7]. In a study of 13 cats treated with radioiodine, serum creatinine rose significantly after treatment [8]. The practical message is that creatinine must be measured before treatment and rechecked after thyroid control is achieved. A cat can look fine on paper while hyperthyroid and then be revealed as having chronic kidney disease once the thyroid is treated. ### Step 7: Thyroid Imaging When Needed Thyroid scintigraphy, which uses a radiotracer to map the gland, shows a heterogeneous pattern of increased uptake, most commonly in both thyroid lobes [1]. It is used to confirm the diagnosis in unclear cases, to find ectopic thyroid tissue outside the neck, and to plan radioiodine dosing [9][10]. ## Diagnostic Workflow The following flowchart shows the main decision path from a suspicious examination to a confirmed diagnosis and treatment planning. ```mermaid flowchart TD A[Older cat with weight loss and good appetite] --> B[Physical exam and weight] B --> C[Total T4 blood test] C --> D{T4 high} D -->|Yes| E[Confirm hyperthyroidism] D -->|No but signs persist| F[Free T4 and repeat total T4] F --> G{Free T4 high} G -->|Yes| E G -->|No| H[Look for other causes] E --> I[Blood count biochemistry urinalysis] I --> J[Blood pressure and kidney values] J --> K[Discuss diet medication surgery or radioiodine] K --> L[Recheck T4 and kidney values after treatment starts] ``` ## Management Options Compared Four treatments are used for feline hyperthyroidism: medical management with methimazole or carbimazole, nutritional management with a low-iodine diet, surgical thyroidectomy, and radioiodine (131I) [11]. Medical and dietary treatments are reversible and palliative, meaning they control the disease while they are used. Surgery and radioiodine are permanent or curative treatments [11]. Each option has advantages and disadvantages that should be weighed for the individual cat and owner [11]. | Treatment | How It Works | Pros | Cons and Cautions | Best Fit | |--|--|--|--|--| | Iodine-restricted diet | Limits iodine so the gland cannot make excess hormone | Non-invasive, no daily pill, avoids some drug side effects | Cat must eat only the prescription diet, no treats or other food. Not all cats accept it | Cats who cannot be medicated or handled, and owners who can strictly control food | | Methimazole, oral | Blocks thyroid hormone production | Reversible, adjustable, widely used, no anesthesia | Daily dosing, possible side effects, requires regular bloodwork, can unmask kidney disease | Cats who tolerate pills and need flexible control | | Methimazole, transdermal | Same drug absorbed through the skin of the inner ear | Avoids pilling, useful for difficult cats | Daily application, skin reactions possible, absorption can vary | Cats who resist oral medication | | Thyroidectomy | Surgically removes the overactive lobe or lobes | Curative, no daily medication afterward | Requires anesthesia and surgery, risk to parathyroid glands, not for all cats | Cats with one clearly affected lobe and good anesthetic candidacy | | Radioiodine (131I) | Destroys overactive thyroid tissue with radiation | Curative, non-invasive, treats ectopic tissue | Requires a referral facility and isolation period after treatment, can cause hypothyroidism and azotemia | Most cats when a facility is available | ## Medical Management With Methimazole Methimazole is the most commonly used antithyroid drug in cats [12]. It works by blocking the production of new thyroid hormone, so it lowers T4 over days to weeks rather than instantly. It is a reversible treatment, meaning the disease returns if the drug is stopped [11]. Methimazole comes as an oral tablet or as a transdermal gel applied to the inner surface of the ear. A prospective study compared oral carbimazole (a drug that converts to methimazole) with a once-daily transdermal methimazole formulation and found both effective at treating hyperthyroidism, with no significant differences in thyroxine concentrations, body weight, blood pressure, heart rate, or key laboratory values over 12 weeks [13]. A separate long-term follow-up study of 60 cats on transdermal methimazole found clinical improvement in all cats, with side effects that were rare and mostly mild, including transient gastrointestinal signs in three cats and ear redness in two cats that required a switch to oral medication [14]. Two practical points about transdermal methimazole come from that long-term study. First, T4 concentrations fluctuated over time, with some cats drifting into the thyrotoxic range and others into the hypothyroid range despite treatment, so regular monitoring matters [14]. Second, the dose needed to keep cats controlled tended to rise over time, with maximum daily doses significantly higher than the starting dose after 24 to 36 months of therapy [14]. That is a reason to recheck rather than assume a stable dose is still working. Timing of blood draws is less critical with transdermal methimazole than you might expect. In a study of 20 cats, T4 suppression was sustained across a 10-hour window after transdermal application, and levels just before the next dose were not significantly different from levels at any point after application, in both once-daily and twice-daily groups [15]. So the exact hour of the blood draw is not the main variable. ### Side Effects of Methimazole Methimazole can cause gastrointestinal upset, liver enzyme changes, and, in some cats, immune-mediated problems. Because both the disease and the drug involve oxidative stress, researchers have studied whether antioxidants reduce drug side effects. In a blinded randomized trial of 30 hyperthyroid cats, cats given methimazole plus antioxidants had lower markers of oxidative stress and a lower rate of methimazole side effects than cats given methimazole plus placebo [16]. A cell-culture study found that methimazole impaired feline kidney epithelial cell viability and increased reactive oxygen species, effects that quercetin counteracted [12]. These findings are promising but do not replace standard monitoring, and any supplement should be discussed with your veterinarian rather than added on your own. Uncommon but serious reactions exist. One case report described a cat that developed pyogranulomatous mural folliculitis, an inflammatory skin condition, 48 hours after methimazole was restarted. The lesions healed and disappeared histologically within 15 days of stopping the drug [17]. Another report described a cat on transdermal methimazole who developed acquired myasthenia gravis, with generalized weakness and regurgitation. Withdrawal of methimazole and treatment with pyridostigmine resolved the signs, and the cat was later controlled with carbimazole plus pyridostigmine without recurrence [18]. If a cat on methimazole develops new weakness, regurgitation, or a sudden skin reaction, an adverse drug reaction should be considered [18]. ## Nutritional Management With an Iodine-Restricted Diet An iodine-restricted prescription diet works by limiting the raw material the thyroid needs to build hormone. If iodine is scarce, the overactive nodules cannot produce excess hormone. In a controlled trial of 34 cats, cats on an iodine-restricted diet as sole therapy had significant drops in total T4, comparable to cats on oral or transdermal methimazole [7]. A key difference emerged in the kidney data: after 90 days, serum creatinine rose significantly only in the methimazole groups, not in the diet group [7]. That does not mean diet protects the kidneys in every cat, but it is a meaningful signal for cats with borderline kidney function. The diet has one strict rule. The cat must eat only the prescription food, with no other food, treats, table scraps, or flavored medications. Any additional iodine source can defeat the treatment. Cats who roam or steal food from other pets are poor candidates. Some cats simply refuse the diet, and that is a legitimate reason to choose a different option. ## Surgical Thyroidectomy Thyroidectomy removes the overactive thyroid tissue. It is a permanent treatment, and in a retrospective study of 41 cats with newly diagnosed hyperthyroidism, cats treated with thyroidectomy had significantly longer survival times than cats treated with methimazole alone [19]. In multivariate analysis, thyroidectomy was an independent predictor of a good outcome, and the recurrence rate was significantly lower after surgery than with methimazole alone [19]. The authors concluded that thyroidectomy is a reasonable irreversible treatment option in settings where radioisotopes are not available [19]. Surgery is not for every cat. It requires general anesthesia, which carries more risk in older cats with heart disease or kidney disease. The parathyroid glands sit right next to the thyroid and control calcium, so they can be injured during surgery, causing low calcium after the operation. Cats with both lobes affected, or with ectopic thyroid tissue outside the neck, are generally better served by radioiodine. ## Radioiodine Therapy Radioiodine (131I) is considered the treatment of choice for feline hyperthyroidism [9][10]. It is given as an injection, and the radioactive iodine concentrates in the overactive thyroid tissue, where it destroys the abnormal cells. It is curative, non-invasive, and it treats ectopic thyroid tissue that surgery would miss. Dosing has improved. Older fixed-dose protocols often caused iatrogenic hypothyroidism and unmasked azotemia. A patient-specific algorithm was developed to calculate the lowest effective dose, using T4 and T3 concentrations, thyroid volume, and the percentage uptake of a radiotracer on scintigraphy [10]. In a series of 1,400 cats treated with this algorithm, 74.8 percent became euthyroid, 4.1 percent became overtly hypothyroid, and the rest fell into intermediate categories [10]. A European series of 165 cats using a similar individualized algorithm reported a median calculated dose of 2.15 mCi, with only 30.9 percent receiving 2.5 mCi or more [9]. Radioiodine has practical requirements. It is done at a referral facility, and the cat must be isolated for a period afterward because of radioactivity. Cats must be off methimazole for at least a week before treatment so the thyroid can take up the radioiodine properly [9][10]. ## The Kidney Problem: Why Treatment Can Unmask CKD This is the single most important concept in managing an overactive thyroid in cats. Hyperthyroidism increases blood flow to the kidneys and raises the filtration rate. That can make a cat with underlying chronic kidney disease look like she has normal kidney values. When thyroid hormone is brought back to normal, kidney blood flow falls and creatinine rises, revealing the disease that was there all along. Two studies show this clearly. In the 34-cat controlled trial, serum creatinine increased significantly after 90 days in both methimazole groups but not in the iodine-restricted diet group [7]. In the 13-cat radioiodine study, serum creatinine rose significantly after treatment [8]. This is not a reason to avoid treating hyperthyroidism. Untreated hyperthyroidism damages the heart and shortens life. It is a reason to measure kidney values before treatment, recheck them after thyroid control is achieved, and adjust the treatment plan if kidney disease appears. Some cats do better with partial control of the thyroid to preserve kidney function, and that is a decision for your veterinarian based on repeated bloodwork. ## Monitoring Schedule After Diagnosis Follow-up bloodwork is not optional. It is how the treatment is tuned. - **Early phase.** Expect a recheck within the first few weeks to confirm the T4 is falling and to catch early drug side effects. - **Stabilization.** Once the T4 is in range, rechecks continue at regular intervals to confirm control holds. - **Kidney values.** Creatinine should be measured before treatment and rechecked after thyroid control is achieved, because of the unmasking effect [7][8]. - **Red cell tracking.** Hematocrit and MCV track T4 within individual cats during methimazole therapy, so a rising hematocrit or MCV can be an early hint that control is slipping and a T4 recheck is due [6]. - **Heart.** Cats with hyperthyroidism should have echocardiographic evaluations before and after treatment to monitor cardiac changes [3]. ## Unsafe Home Remedies and Common Mistakes - **Do not give your cat human thyroid medication or any human antithyroid drug.** These are not dosed for cats and can cause serious harm. - **Do not add iodine supplements or kelp.** Adding iodine to a cat with an overactive thyroid can feed the disease. If your cat is on an iodine-restricted diet, any iodine source defeats the treatment. - **Do not rely on diet alone without telling your veterinarian.** The diet only works if it is the sole food source, and it still requires monitoring. - **Do not stop methimazole abruptly without guidance.** Stopping treatment lets thyroid hormone climb back up. - **Do not skip rechecks because your cat looks better.** Clinical improvement happens before laboratory control is confirmed, and kidney values can change after treatment starts. - **Do not assume a stable dose stays stable.** In long-term transdermal methimazole treatment, required doses rose over time, and T4 drifted in both directions [14]. - **Do not use unproven supplements as a substitute for treatment.** Antioxidant research is promising for reducing drug side effects, but it does not treat the disease itself [16]. ## Prevention and Risk Reduction There is no proven way to prevent feline hyperthyroidism. The disease arises from nodules with excessive intrinsic growth capacity, and the roles of iodine deficiency, nutritional goitrogens, and environmental disruptors are not fully established [1]. What owners can control is early detection. Because the disease is strongly age-linked and most affected cats are seniors [2], routine senior wellness visits with a T4 test are the most effective tool. Any cat over 6 years old with signs of heart disease should be screened for hyperthyroidism [3]. Catching the disease before the heart and kidneys are affected makes every treatment option easier. ## Prognosis The prognosis for a cat with hyperthyroidism is good when the disease is diagnosed and treated. Clinical signs improve with every treatment option [14][11]. Cardiac changes often reverse once thyroid hormone is normalized [3]. In one study, thyroidectomy was associated with longer survival than methimazole alone, and with a lower recurrence rate [19]. Radioiodine produces a cure in most cats, with a small percentage becoming hypothyroid and requiring thyroid supplementation [10]. The main threats to a good outcome are late diagnosis, untreated heart disease, and unrecognized kidney disease that appears after thyroid control. Regular monitoring addresses all three. ## Emergency Red Flags Seek emergency care immediately if your cat has any of these: - Open-mouth breathing, labored breathing, or blue gums - Collapse, fainting, or sudden inability to stand - A heart rate that feels extremely fast or irregular - Severe weakness or a sudden inability to use the back legs - Repeated vomiting that prevents eating or drinking - Seizures or sudden disorientation - Sudden severe skin lesions, facial swelling, or difficulty swallowing while on methimazole ## Limitations and When to Contact a Veterinarian This article describes general patterns. Your cat's case depends on her age, heart, kidneys, other medications, and how she tolerates each treatment. Only your veterinarian can examine her, interpret her bloodwork, and set a plan. Contact your veterinarian if: - Your cat has weight loss with a normal or increased appetite for more than a week - You notice increased drinking or urination that is new - Your cat is vomiting repeatedly or has persistent diarrhea - Your cat seems restless, vocal, or irritable in a way that is new - Your cat is on methimazole and develops weakness, regurgitation, skin lesions, or vomiting - Your cat is on an iodine-restricted diet and you cannot keep her from eating other food - Your cat's breathing rate at rest increases, or she seems to tire easily - A recheck blood test shows rising creatinine or a T4 outside the target range ## Frequently Asked Questions ### What are the first signs of an overactive thyroid in cats? Weight loss with a normal or increased appetite is the classic first sign. Increased drinking and urination, vomiting, restlessness, and a poor coat often appear around the same time. ### Can a cat with hyperthyroidism be cured? Yes. Radioiodine and surgical thyroidectomy are permanent treatments that can cure the disease. Methimazole and an iodine-restricted diet control it but do not cure it, so the disease returns if treatment stops. ### Is a low-iodine diet as effective as medication? In a controlled trial, an iodine-restricted diet lowered T4 as effectively as oral or transdermal methimazole over 90 days. The diet only works if the cat eats nothing else, and it still requires monitoring. ### Why did my cat's kidney values get worse after starting treatment? Hyperthyroidism increases kidney blood flow and can hide chronic kidney disease. When thyroid hormone is brought down, kidney values can rise and reveal the underlying disease. This is expected and is managed with monitoring, not by avoiding treatment. ### How often does my hyperthyroid cat need bloodwork? Expect a recheck in the first few weeks after starting treatment, then regular rechecks once the T4 is in range. Creatinine should be measured before treatment and after thyroid control is achieved. ### Can I give my cat treats while on the iodine-restricted diet? No. The diet only works when it is the sole source of food. Treats, table scraps, and flavored medications can add iodine and defeat the treatment. ### Is transdermal methimazole as good as the pill? A 12-week study found once-daily transdermal methimazole as safe and effective as oral carbimazole. Long-term, transdermal treatment works well in most cats, though ear redness can occur and doses may need to rise over time. ### What happens if hyperthyroidism is left untreated? Untreated hyperthyroidism keeps the heart working too hard and can lead to heart failure. One case report described congestive heart failure in a cat with chronic, poorly controlled hyperthyroidism. Treatment prevents this. ## Related Articles - [What Are the Side Effects of Thyroid Medication for Dogs?](/knowledge/veterinary-medicine/pet-medication-guide/dog-thyroid-medication-side-effects) - [Can Dogs Eat Tofu? Soy Phytoestrogen and Thyroid](/knowledge/veterinary-medicine/toxicology-food-safety/can-dogs-eat-tofu-soy-phytoestrogen-and-thyroid) - [What to Expect When Your Cat Needs a Thyroid Medication Adjustment](/knowledge/veterinary-medicine/pet-medication-guide/cat-thyroid-medication-adjustment) - [What to Do If Your Cat Misses a Dose of Thyroid Medication](/knowledge/veterinary-medicine/pet-medication-guide/cat-missed-thyroid-medication) - [Thyroid Testing in Cats: Diagnosing Hyperthyroidism and T4 Fluctuations](/knowledge/veterinary-medicine/at-home-diagnostics/thyroid-testing-in-cats-diagnosing-hyperthyroidism-and-t4-fluctuations) - [What to Do If Your Cat Refuses to Take Its Thyroid Medication](/knowledge/veterinary-medicine/pet-medication-guide/cat-refuses-thyroid-medication) - [Dog Gestation: Timeline, Signs, and Care](/knowledge/veterinary-medicine/theriogenology/dog-gestation-timeline-signs-and-care) - [FeLV Symptoms in Cats: Stages and Warning Signs](/knowledge/veterinary-medicine/symptom-guides/felv-symptoms-in-cats-stages-and-warning-signs) ## Sources 1. 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