Hyperthyroidism involves over-secretion of thyroid hormones and is the most common hormone imbalance in cats.
What is hyperthyroidism?
All cats have two thyroid glands, one on each side of their windpipe that produce a hormone called T4. The body then converts this T4 to the T3 hormone which is required for basic metabolic function.
In Hyperthyroidism, a benign growth occurs in one of the glands causing too much T4 hormone to be produced. Very rarely (3-5% of cases) these growths are cancerous.
The most common symptom we see is cats that are losing weight but have a persistently huge appetite.
Other symptoms may include increased drinking or urinating, scruffy coats or a change in behavior. We also often see vomiting or diarrhea. The vet may also notice a heart murmur and enlarged thyroid gland on physical examination.
HYPERTHYROIDISM IS GENERALLY A DISEASE OF OLDER CATS.
THE AVERAGE AGE AT DIAGNOSIS IS ABOUT 13.
This is achieved by a simple blood test which is usually sent externally so a full panel can be checked for associated disease (e.g. liver, kidney, white blood cells, protein levels and red bloods cells). If the Total T4 level is elevated on the blood test, diagnosis is confirmed. Some cats may be borderline and require repeat or further testing.
This method of therapy is generally considered the safest and most effective method of treatment for feline hyperthyroidism. This involves sending the cat to a nuclear medicine facility where it receives an injection of a radioactive substance, pertechnetate to confirm diagnosis via a scan. Based on these findings the cat receives iodine 131 treatment which is a radioactive isotope that destroys the abnormal thyroid tissue via high speed electron emission.
This treatment rarely requires a repeat course and no additional therapy is required. Cats must stay hospitalized for an extended period (up to 10 days) whilst they are exposed to the radiation treatment. After the cat comes home, some restrictions must be imposed for another week or so and the cat must use special flushable litter, not be allowed outside, reduce owner contact and children and pregnant women should not come into contact with them.
Blood work is monitored following treatment to ensure return to normal thyroid status. Occasionally, a single course of radiotherapy is inadequate and a second course is needed.
- Treatment is a one-time event (only 2 to 4% of cats require a second treatment) and no on-going therapy is required.
- The disease is not simply managed but is actually cured!
- No anaesthesia is required, indeed, treatment amounts to an injection followed by 3 to 7 days of boarding, very non-stressful for older cats with potential heart disease.
- If a cat is one of the unlucky 3 to 5% for which the thyroid tumour is malignant, the initial pertechnetate scan will indicate this right away.
- Owner and pet are separated during the quarantine.
- Some facilities require the cat to be confined indoors or have limited contact with owners for a period of time after discharge from the facility. Children and pregnant women can have no contact with the cat for a week or two after therapy. If this is too inconvenient to work out at home, the cat may be boarded until this period has passed.
- This is a relatively expensive therapy e.g. upwards of $1700
- Special flushable cat litter is required for 1 to 2 weeks after therapy.
- Some follow-up blood testing is generally recommended after treatment (typically 1 and 3 months after therapy).
- There is a chance (less than 5%) that the cat will become HYPOthyroid after treatment, requiring daily oral thyroid hormone supplementation.
- Radiotherapy may not be a good idea for a cat with poor kidney function or may in fact unmask poor kidney function once therapy is completed.
The goal here is to remove the abnormal thyroid tissue, leaving the normal adjacent tissue alone. This is a complicated surgery that is performed by specialist surgeons so your vet will refer you if you are considering this treatment option. Not all hyperthyroid cats are surgical candidates so your vet will discuss whether it is an option. This surgery is rarely recommended these days considering there are less invasive alternatives available.
- Treatment is generally permanent after recovery. Unless complications arise, no further treatment is needed.
- Performing surgery and general anaesthesia on a geriatric patient with potential heart disease has inherent risk.
- If abnormal thyroid tissue is left behind, hyperthyroidism is likely to recur within 6 to 24 months Continued monitoring of T4, typically twice a year, is frequently recommended.
- The monitoring before and after surgery, plus the surgery itself, is relatively expensive.
- The laryngeal nerve is located near the thyroid gland. If it is damaged during surgery, the cat can experience a voice change. This change may be permanent.
- The sympathetic trunk (another neurologic tissue) is located near the thyroid gland. If it is damaged during surgery, the eye on that side may develop what is called Horner’s syndrome. This syndrome may be permanent.
- Treatment may unmask poor kidney function.
- It takes 1 to 3 months for thyroid blood levels to stabilize following surgery. Some cats become hypo thyroid after surgery and must take thyroid supplementation tablets either temporarily or permanently.
- Parathyroid gland damage causing calcium depletion.
The most common medication prescribed to treat feline hyperthyroidism is called methimazole or carbimazole. This medication will need to be given at least once daily for the remainder of their life.
These medications block the production of T4 and T3. Thyroid hormones that are already in the body when medication is started are still in play, so a good 2 to 4 weeks are needed before thyroid blood tests will show the effect of treatment.
It is important to understand what the potential side effects of methimazole and its relatives are, and the monitoring that is generally recommended. The thyroid nodule, which can be detected in a cat’s throat, will not reduce in size with treatment and may in fact get larger.
- Medication is relatively inexpensive.
- Control of thyroid disease is achieved only while the pet is on medication so that if there is any problem with exacerbated poor kidney function, treatment can be discontinued.
- No hospitalization is required.
- Side effects are relatively uncommon.
- If an occasional dose is skipped, no harm is done.
- If no side effects are encountered after the first 3 months of therapy, the chance of side effects occurring thereafter is substantially reduced.
- Medication must be given at least daily (usually twice daily). Some cats simply will not take oral tablets at this frequency. Methimazole is readily made into a flavoured liquid or transdermal paste for easier administration by a compounding pharmacy.
- Approximately 15% of cats will experience some kind of side effect. The usual side effects are: lethargy, loss of appetite, and vomiting. If one of these side effects occurs, medication is discontinued until the symptoms resolve. Medication is then restarted at a lower dose and gradually increased to the former dose. These side effects do not generally recur if medication is increased gradually in this way.
- Facial itching is a more serious side effect. This side effect also resolves with anti-itch medication and discontinuing methimazole. Cats who have this side effect can be expected to have it again if medication is restarted, so another form of treatment should be used. Facial itching occurs in less than 4% of cats on methimazole.
- Serious liver failure results in an extremely small number (less than 2%) of cats taking methimazole. This toxicity can be expected to resolve after discontinuation of the medication but, again, alternative therapy should be considered.
- Bone marrow changes can also result from methimazole administration. Blood tests evaluating white blood cell patterns should be periodically performed to monitor for these changes. This side effect occurs in less than 4% of cats on methimazole and necessitates a change in therapy.
- Pre-existing kidney insufficiency can be masked in hyperthyroidism because the heart disease and high blood pressure that go with hyperthyroidism increase blood flow through the kidneys, making the kidneys more efficient. Once treatment is instituted for hyperthyroidism, the kidney disease is unmasked or made worse when kidney blood flow returns to normal. Sometimes it is necessary to choose between treating the kidneys and treating the thyroid, thus monitoring kidney function and thyroid levels is particularly important during methimazole therapy. Kidney problems can be minimized by starting with a lower dose of methimazole and working up over weeks or months so as not to cause as abrupt a change in kidney blood flow. If kidney problems become significantly worse on methimazole, medication can be discontinued. Approximately 15-22% of cats treated for hyperthyroidism will show kidney disease that was not evident prior to treatment.
MOST SIDE EFFECTS OCCUR DURING THE FIRST 3 MONTHS OF METHIMAZOLE THERAPY.
Periodic blood testing to examine T4 level, white blood cell patterns, kidney function, and liver enzymes should be periodically performed. Be sure to ask your veterinarian to review an appropriate schedule for your cat. Side effect potential can be reduced by beginning at a smaller dose and working up to the full therapeutic dose over the first couple of months of therapy.
Hills have recently developed a hyperthyroidism diet that aims to reduce iodine in their diet which they need to produce thyroid hormones. Less iodine means their thyroid hormones drop and their thyroid acts normal again. The food also has appropriate sodium and phosphate levels for older cats that potentially have concurrent kidney issues. Feeding this food exclusively means no other treatment is required. However NOTHING else can be fed and the cat must have absolutely no access to other meal sources. They must also eat out of certain types of bowls to ensure no iodine is apparent in the plastic. The food is prescription only and thus relatively expensive however is a great option for patients that are hard to medicate.