The Thyroid Glands

The main thyroid glands are located in the neck – there is one on the left and one on the right of the trachea. There may also be some thyroid tissue further down the neck and into the chest too.  The thyroid glands produce thyroid hormones T4 (thyroxine) and T3 (triiodothyronine) which:

    • Are essential for proper growth of body cells and the development of these cells for their specific roles in the body
    • Help regulate metabolism of protein, fat and carbohydrate by cells
    • Are involved in the regulation of heat production and oxygen consumption and therefore a wide range of metabolic processes
    • In other words influences the speed at which your body cells work

Hyperthyroidism is a common condition in cats from approximately 10 years upwards but occasionally in younger cats. It causes excess thyroid hormone to be produced usually from a benign adenoma of the thyroid gland or glands – in about 70% of cases both glands are affected but one is usually worse than the other or develops first.  Excessive thyroid hormone leads to a dramatic increase in the cat’s metabolic rate and affects the function of virtually every organ system.  It is rare for cats to develop cancerous growths of the thyroid gland.

Signs of Hyperthyroidism

  • Weight loss is the most common sign despite usually having an increased appetite (“polyphagia”)16394609 - feed me
  • Unkempt coat
  • Increased drinking and urinating (“polydipsia & polyuria”)
  • Hyperexcitability and changes in behaviour
  • Some gastrointestinal signs, such as vomiting, diarrhoea or increased faecal volume
  • An enlarged thyroid gland – which can often be felt in the neck by your vet
  • Increased heart rate
  • Enlarged heart and if left undiagnosed congestive heart failure causing difficulty breathing
  • Rarely hyperthyroid cats can be depressed and lack appetite but they will still loose weight


This is normally quite simple with a blood test to measure the levels of the T4 hormone. Occasionally a second thyroid blood test is needed.

Often a full wellness screen is done to check that there is nothing else wrong with your older cat before any treatment is started.  Although kidney disease is not directly linked to hyperthyroidism both may develop at the same time and are common in older cats.  Managing hyperthyroidism can sometimes have adverse effects on kidney function so it is useful to know the status of the kidneys before starting treatment.


Hyperthyroidism can be treated very successfully by different methods of bringing the levels of thyroid hormone in the blood back to normal.


Tablets called Felimazole or Vidalta will be prescribed for your cat to take every day.  These are usually used to stabilise your cat but can be used for long term treatment too.  The active drug in these tablets is Methimazole, which interferes with the synthesis of thyroid hormones – it usually takes 2-3 weeks for levels to return to normal (“euthyroidism”) but sometimes higher doses are needed.

If continuing on tablets long term with no other treatment your cat will need daily tablets for the rest of their life. Blood samples are needed to monitor levels of T4 hormone to adjust dosing if necessary.  Other blood tests may be included to monitor the health of your cat too as it gets older, especially of the kidneys.

Occasionally cats can get side effects from these tablets. Vomiting and decreased appetite can be short term when treatment is started but then return to normal.  Rarely cats can become itchy or have haematological (blood cell) abnormalities, such as decreased white blood cells.


Often, after initial stabilisation with tablets as above, cats will undergo surgery to remove the affected thyroid glands – a procedure called a thyroidectomy.  This involves a general anaesthetic and generally an overnight stay with us at the practice.  Your cat will have a small wound on the underside of their neck, normally with 2 or 3 stitches, which can be removed 10 days after the surgery.  Once this is complete your cat will no longer need tablets although as previously mentioned, some cats will go on to develop over activity in the gland on the other side of the neck.

We stabilise cats first to ensure their heart rate is more normal and also check kidney parameters are normal before a general anaesthetic is done. The main risk of the surgery itself is damage to the parathyroid glands, which are small glands that lie close to or within the thyroid glands themselves.  They have a crucial role in maintaining stable blood calcium levels – therefore temporary interference with calcium regulation may occur.  The risk is low when a single thyroidectomy (on one side only) is being performed but will be higher if a second (on the other side) is performed later in life.  We do not normally remove both thyroid glands at the same time.  Cats having a second thyroidectomy may be hospitalised for longer and need blood calcium concentrations monitored after surgery.

Radioactive Iodine Therapy

Radioactive iodine (I-131) is a very safe and effective treatment for hyperthyroidism but involves your cat being hospitalised for 2-4 weeks in a specialist centre. The main advantage is that the treatment is curative in most cases with no ongoing treatment after a single injection, usually given under the skin.  The iodine targets the abnormal tissue by being selectively taken up by the active (abnormal) thyroid tissue and accumulating here so the radiation destroys this tissue.  It is not taken up by any other normal tissues and does not damage the surrounding tissue or parathyroid glands.  There are no significant side effects but because cats are temporarily radioactive they have to be hospitalised at specialist centres.

95% of cats are cured after a single injection but occasional blood tests are recommended to ensure normal thyroid hormone levels are being maintained. Rarely cats may become hypothyroid (thyroid hormone levels too low) but this is normally temporary if it does occur.

If you have any questions or think your cat may be suffering for hyperthyroidism, don’t hesitate to call us or please book an appointment with one of our vets by calling:

Malvern: 01684 572420

Upton-upon-Severn: 01684 593253