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Managing your Feline Hyperthyroid Patients with Nutrition



Do you see and manage many hyperthyroid cats in your practice?


I’m guessing the answer is probably yes, given hyperthyroidism is recognised as the most common endocrinopathy of older cats.  Traditional methods of managing feline hyperthyroidism include thyroidectomy, anti-thyroid medications, and radioactive iodine. In 2016, The American Association of Feline Practitioners (AAFP) released guidelines for the management of feline hyperthyroidism which included dietary management, with Hill’s™ Prescription Diet™ y/d™, as a management option.  The guidelines are well worth a read as they summarise the advantages and disadvantages of all 4 management options that are currently available for hyperthyroid felines.

How does y/d work?

Excessive production of thyroid hormone is the hallmark of feline hyperthyroidism. Production of thyroid hormone requires uptake by the thyroid gland of sufficient amounts of iodine, which is provided by dietary intake.1 This prompted the hypothesis that limiting dietary iodine intake could be used to control thyroid hormone production and manage hyperthyroidism in cats. After more than a decade of research and development, a limited‐iodine therapeutic food, Hill’s™ Prescription Diet™ y/d™, was made available as an option for managing cats with hyperthyroidism. This video also explains how it works.

Thyroid hormone

Clinical Studies Supporting Nutritional Management of Feline Hyperthyroidism

A prospective, multi-centre, non-controlled, open-label study evaluated the effects of an iodine-restricted food (Hill’s™ Prescription Diet™ y/d™) on circulating total thyroxine (TT4) concentrations and clinical parameters in 225 client owned hyperthyroid cats from 139 veterinary clinics in 10 European countries.2  This study showed that eating an iodine-restricted food is a valuable management option, able to decrease plasma TT4 concentrations and improve clinical signs and quality of life in these cats after 4-8 weeks.2   Interestingly, no difference,  in response to the iodine restricted food, was found between indoor versus outdoor cats,  suggesting that the possible ingestion of prey was not sufficient to negate the effectiveness of  y/d™.2


In another study, 49 client-owned cats with spontaneous hyperthyroidism were evaluated in a retrospective case series over one year.3 In this study, 42% and 83% of cats had serum TT4 concentrations within reference range after exclusive feeding of the restricted-iodine diet (Hill’s Prescription Diet™ y/d™) by 60 days and 180 days, respectively.3  Importantly, this study found that the cats whose TT4 took longer to return to the reference range were more likely to have a higher initial TT4 concentrations compared to those cats whose TT4 was within range by day 60, indicating that normalisation may require up to 180 days in cats with severe elevations in TT4.3


Three earlier studies have also documented the safety and efficacy of feeding  y/d™ as the sole means of managing cats with naturally occurring hyperthyroidism.4-6 These studies were designed to determine: the magnitude of iodine restriction necessary to return newly diagnosed hyperthyroid cats to a euthyroid state;6 the maximum level of dietary iodine that will maintain cats in a euthyroid state;5 and the efficacy of a therapeutic food formulated based on the previous studies to control naturally occurring hyperthyroidism in cats.7  These studies indicate that a therapeutic food with dietary iodine levels at or below 0.32 ppm dry matter basis (DMB) is an effective and safe therapy.

Potential Reasons to Consider Nutritional Management of Feline Hyperthyroidism

Owner Factors
Older cats
  • Radioactive iodine not available or the owner does not desire this option
  • Owner reluctant to have cat away during hospitalisation period for radioactive iodine
  • Costs of radioactive iodine treatment or surgery are prohibitive for the owner
  • Owner capable of feeding only limited-iodine food as the sole source of nutrition
  • Owner cannot comply with giving medications (oral or transdermal)
  • Delivering frequent medication has negative impact on the family-pet bond
  • The cat has had an adverse reaction to anti-thyroid drugs
  • Irreversible management not appropriate (e.g., concerns about unmasking or worsening early to moderate kidney disease)
  • Thyroidectomy not appropriate (e.g., high anaesthetic risk due to cardiac disease, bilateral disease with higher risk of parathyroid damage)
  • Cat has early to moderate kidney disease

Nutritional Management of Hyperthyroid Cats with Concurrent Chronic Kidney Disease

Chronic kidney disease is the most common concurrent condition of hyperthyroid cats. Both diseases have increased prevalence in geriatric patients.  In addition to demonstrating the efficacy of  y/d™ in restoring euthyroidism, three studies have showed reductions in serum creatinine concentrations together with stable or increasing bodyweights in hyperthyroid cats eating the diet.2,3,7   Whilst the mechanisms behind these effects are currently unknown the findings are significant, as a decrease in renal function is commonly seen after successful management of hyperthyroidism in cats using other treatment methods.8,9   When compared with typical feline foods, y/d™ contains controlled amounts of phosphorus and sodium, high quality/ highly digestible protein, and is supplemented with omega‐3 fatty acids. Additional study is needed to better understand the effects of feeding y/d™ to hyperthyroid cats with concurrent kidney disease, and hyperthyroid cats with kidney disease should be monitored closely regardless of which option is selected for managing their hyperthyroidism. The aforementioned AAFP guidelines also provide information on how to manage feline hyperthyroidism in the presence of other non-thyroidal disease such as underlying kidney disease and conclude that dietary management with an iodine-restricted food is an option for cats with concurrent renal insufficiency with appropriate monitoring.

Nutritional Management of Newly Diagnosed Cats

After confirming the diagnosis and performing a thorough patient evaluation, all management options should be discussed with owners. If nutritional management is selected, gradual transition to y/d™ over at least 7 days is recommended. Based on palatability testing and feedback received from veterinarians and pet owners, acceptance of the food is very good. Gradual transition is still recommended to decrease side effects (inappetence, vomiting, diarrhoea, flatulence) that can occur with any food change. Some cats may benefit from a transition over several weeks. It is very important to counsel owners so they understand that success of nutritional management depends on the limited‐iodine food being the sole source of nutrition. The first recheck evaluation should be done 4 weeks after completing the transition to y/d™ (i.e., once the cat has eaten y/d™ exclusively for 4 weeks) and as a minimum should include physical examination and measurement of TT4, BUN, serum creatinine, and urine specific gravity. Cats should have decreased TT4 concentrations compared with baseline by the 4‐week evaluation, and there will be cats whose TT4 has returned to normal by this time. Clinical improvement, including weight gain, improved hair coat and decreased tachycardia/cardiac murmur, may also be noted by the first evaluation. Clinical signs should continue improving, and by the next re‐evaluation at 8 weeks, most cats will be euthyroid if y/d™ is their sole source of nutrition, with a small minority of cats requiring slightly longer.

A group of cats

Transitioning from Anti-Thyroid Drugs

Concurrent use of anti-thyroid drugs and feeding y/d™ is NOT recommended because of the potential for causing a rapid decrease in TT4 concentration and hypothyroidism.  If the decision is made to transition a hyperthyroid cat from anti-thyroid drugs to a limited-iodine food, a baseline evaluation (history, physical examination, CBC, serum chemistries, urinalysis, and TT4) should be done prior to beginning nutritional management.  This is very important for identifying concurrent diseases and establishing a base from which to evaluate effectiveness/impact of nutritional management. It should not be assumed that cats receiving anti-thyroid medication have normal TT4 concentrations.  Some cats that are well regulated with anti-thyroid drugs may experience a transient increase in TT4 after switching to nutritional management (rarely associated with a recurrence of clinical signs), which usually resolves once the cat has been eating y/d™ as the sole source of nutrition for 1-2 months.  Monitoring is indicated as described for newly diagnosed patients.

Guidelines for Managing Cats with Persistent Hyperthyroidism

The majority of hyperthyroid cats managed with Hill’s™ Prescription Diet™ y/d™ as the sole source of nutrition became and remained euthyroid as long as the cat had no access to other sources of dietary iodine. Persistently increased TT4 should raise concerns of poor adherence to dietary recommendations. Maintaining y/d™ as the sole source of nutrition can be challenging for some owners, and discovering the source of dietary iodine intake can be challenging for the veterinary health care team.  In keeping with the principles of client centred communications, owners should be questioned in an open-ended, non-threatening manner.  Appendix 1 lists a series of questions, the rationale for each question, and actions that can be taken to rule out sources of dietary iodine intake.


Hyperthyroidism is considered the most common endocrine disease of older cats worldwide and several effective management options are available. All options should be discussed with pet owners, including pros and cons, so that the best can be selected for individual patients and their owners. Hill’s Prescription Diet™ y/d™ is the first therapeutic food specifically formulated to manage hyperthyroidism in cats. Studies document that when y/d™ is fed as the sole source of nutrition, it is a safe and effective. Management of feline hyperthyroidism can be as easy as feeding your patient.


Have questions? Check out our Frequently Asked Questions Document or call our Veterinary Consultation Service on 1800800733.

Dr Jennifer Ervin BVSc (Hons), Professional Consulting Veterinarian, Hill’s Pet Nutrition Australia HIMA-JE-1825A79C

Jen graduated from the University of Melbourne with honours in 1999.  After graduation Jen worked in mixed animal/dairy practice for a short spell before moving into 100% small animal practice.  In 2002, Jennifer and her now husband Matthew (also a veterinarian), spent two years working in the U.K.  This entailed many different small animal veterinary roles including work in an emergency centre, as well as a stint as a greyhound track vet! Since returning to Australia, Jen has worked as a sole charge practitioner in small animal practice, spent a year as a Veterinary Territory Manager for Hill’s and also worked as a Practice Manager of a large mixed animal practice for eighteen months.  Since 2008, Jennifer has worked as a Technical Services Veterinarian for Hill’s Pet Nutrition

Dr Jennifer Ervin


  1. Mooney CT. Hyperthyroidism In: Ettinger SJ, Feldman EC, editors. Textbook of Veterinary Internal Medicine: Diseases of the Dog and Cat. 7th edn. Saunders, St. Louis, MO.2010;1761-1779.
  2. van der Kooji M, Becvarova I, Meyer HP et al. Effects of an iodine-restricted food on client-owned cats with hyperthyroidism. J Feline Med Surg 2014; 16: 491-498.
  3. Hui TY, Bruyette dS, Moore GE, et al. Effect of feeding an iodine restricted diet in cats with spontaneous hyperthyroidism. J Vet Intern Med 2015; 29: 1063–1068.
  4. Melendez LM, Yamka RM, Burris PA. Titration of dietary iodine for maintaining normal serum thyroxine concentrations in hyperthyroid cats (abstract) J Vet Intern Med 2011;25:683.
  5. Melendez LM, Yamka RM, Forrester SD et al. Titration of dietary iodine for reducing serum thyroxine concentrations in newly diagnosed hyperthyroid cats (abstract). J Vet Intern Med 2011;25:683.
  6. Yu S, Wedekind K, Burris PA et al. Controlled level of dietary iodine normalizes serum total thyroxine in cats with naturally occurring hyperthyroidism (abstract). J Vet Intern Med 2011;25:683.
  7. Fritsch d, Allen T, Dodd C, et al. A restricted iodine food reduces circulating thyroxine concentrations in cats with hyperthyroidism. Intern J Appl Res Vet Med 2014; 12: 24–32
  8. Boag AK, Neiger R, Slater L et al. Changes in the glomerular filtration rate of 27 cats with hyperthyroidism after treatment with radioactive iodine. Vet Rec 2007;161:711-719
  9. Syme HM. Cardiovascular and renal manifestations of hyperthyroidism. Vet Clin Small Anim Pract 2007;37:723–743

Appendix 1: Suggestions for questions for clients with cats with persistent hyperthyroidism


Question Guide