Nutritional management of feline hyperthyroidism

Feline hyperthyroidism is a common condition typically affecting older cats and the clinical signs can be quite serious, causing significant illness. As the condition is caused by the thyroid overproducing the hormones T3 and T4 which are also responsible for controlling the body’s metabolic rate, significant weight loss and increased appetite is the most common symptom reported. Hyperthyroidism can be treated with medications, surgery or radioactive iodine therapy, but diet is often the forgotten treatment. With surgery comes risk, and with daily medications brings stress for the pet owner, so diet can be a great choice for these cats with a ravenous appetite and typically co-morbidities.

So what causes feline hyperthyroidism and how is it managed with diet?

While the pathology and treatments of feline hyperthyroidism are well documented, the causes and risk factors are not fully understood. Some risk factors and triggers have been documented to be:

  • Development of thyroid nodules or tumours causing overproduction of thyroid hormone
  • Genetic component that leads to excessive growth of the thyroid
  • Consumption of canned foods, especially fish or liver flavours
  • Exposure to a variety of environmental contaminants; some types of cat litter, herbicides or insecticides appear to increase risk of developing the condition
  • Nutritional deficiencies or excesses

Dietary considerations

The main dietary considerations we need to think about when feeding hyperthyroid cats is avoiding feeding excessive levels of iodine-containing compounds or diets. Iodine content in commercial food is highly variable and not usually something that is reported or tested by the manufacturer, but there are things we can look for that will be safer. The reason canned foods are often avoided is due to the radiation and environmental contaminants that can be potentially harmful to thyroid hormone synthesis. In hyperthyroid cats, we need to consider each cat’s situation on a case by case basis when deciding on what to feed them.

Foods & compounds that can affect thyroid function

  • Cabbage (goitrin)
  • Canned foods
  • Cassava (linamarin)
  • Cyanides
  • Millet
  • Quercetin
  • Rutabagas
  • Selenium
  • Soy
  • Sweet potatoes
  • Turnips
  • Seaweed
  • Various beans (including soybeans)
  • Polyphenols (fish-containing foods)
  • Resorcinols (fish-containing foods)
  • Polychlorinated biphenyls (PCB, fish-containing foods)

If feeding a home-cooked diet, these ingredients should be avoided.

Nutritional management of thyroid disease

I break down nutritional management into three categories:

  • Unmedicated cats
  • Medicated cats and symptomatic cats with comorbidities
  • Cats recovering from thyroidectomy or radioactive iodine treatment

Unmedicated cats:

These cats may be unable to be treated with medications for a variety of reasons – either they are too difficult to medicate, or the owner is not comfortable with or financially able to medicate the cat due to personal circumstance. Thankfully, we have a therapeutic diet that can successfully and safely manage feline hyperthyroidism. Hills Prescription Diet Y/d is a low iodine diet designed to induce a normal thyroid state after about 8 – 12 weeks of being fed as the sole diet (most patients will see a significant decrease by week 3). It is most effective in cats with moderate elevations of T4 though is still useful in cats with severe hyperthyroidism, it just will take significantly longer to lower T4 levels. It is also designed to manage other common co-morbidities such as renal and cardiac disease due to its controlled phosphorus and sodium levels. However, if the patient has thyroid tumours, it will not ‘cure’ the disease – it will only suppress hormone production. It’s important to note that Y/d should only be used in unmedicated cats – never in conjunction with medications; some clinicians may opt to feed Y/d for a short period to lower thyroid levels before transitioning to medications or having radioactive iodine therapy for long term control.

Medicated cats and symptomatic cats with comorbidities:

Another method of nutritional management is focusing on addressing the symptoms and associated conditions of feline hyperthyroidism. Cats with hyperthyroidism often have muscle wastage and weight loss despite significant dietary intake, poor hair coat, excessive thirst, diarrhea and/or vomiting. All these symptoms can be improved with diet; gastrointestinal foods are usually higher in calorie content and can help address the gastrointestinal signs. If the cat suffers from an associated condition, such as renal or cardiac disease, feeding a diet for these conditions while medicating for hyperthyroidism is a really great way to manage all conditions, rather than having to make a decision between one or the other.

Recovering from thyroidectomy or radioactive iodine treatment:

As with any patient recovering from a hospital stay, these events can be stressful and impact the body quite significantly. In these patients, keeping them calm and relaxed is incredibly important as stress reduces the body’s ability to heal effectively. Feeding a recovery diet can be useful in providing all the necessary nutrients in allowing the body to heal and keeping them eating while also providing extra calories for these pets who are so susceptible to weight loss and muscle cachexia. However if the cat is happy eating their current diet after their hospital stay, don’t change it – if we need to change the diet due to a different or related condition, wait at least two weeks after the hospital before transitioning to avoid developing food aversions.

Hyperthyroid cats have wide and varying needs, so always seek the opinion of your vet or nutritionist before changing or supplementing their diet. Do you care for a hyperthyroid cat? How do you manage their nutritional needs? Leave us a comment below!


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