Hyperthyroidism is the most common hormonal disorder in cats. Ten percent of all cats over 13 years of
age are affected. In 98% of cases, the cause is a benign thyroid tumor (adenoma); only 2% of hyperthyroid cats suffer from a malignant adenocarcinoma.
Both tumor types produce excessive amounts of thyroid hormones, which act on all organ systems in the body and gradually lead to a metabolic imbalance.
The classic symptoms of hyperthyroidism include hyperactivity up to aggressiveness, weight loss despite
a voracious appetite, and gastrointestinal problems such as vomiting or diarrhea.
Cardiovascular complications often include high blood pressure affecting sensitive organs such as the kidneys (kidney failure), retina (acute blindness), and brain.
If hyperthyroidism is left untreated, it consequently leads to thickening of the heart muscle with fatal outcomes such as pulmonary edema, aortic thrombosis, or heart failure.
Therapy for feline hyperthyroidism therefore greatly improves the quality of life of affected cats and often extends the lives of our thyroid patients by several years.
Available Treatment Options:
1. Daily oral treatment with antithyroid drugs (tablets or liquid):
This form of therapy only regulates the symptoms. The underlying cause – the
hormone-producing tissue – persists and continues to grow.
Adenomas can develop into malignant carcinomas, and so-called “SHIM-RAD” tumors may occur
(Severe elevations of T4, Huge tumor size, Intrathoracic nodules, Multifocal disease, and Refractory to treatment with antithyroid drugs).
In such cases, the symptoms can no longer be controlled and may lead to the cat’s death.
Disadvantages include not only the need to administer medication twice daily but also
possible side effects such as skin changes, thrombocytopenia, and hepatopathies.
2. Transdermal treatment with antithyroid drugs:
Same principle as oral administration. Its efficacy depends on absorption, and cats may additionally develop local skin irritation at the application site as a side effect.
3. Iodine-restricted diet (Hill’s y/d):
By reducing the iodine content, this diet decreases the production of thyroid hormones.
Feed acceptance plays a major role, as the cat must eat this food exclusively in order for it to be effective.
4. Surgical removal of the thyroid gland:
An invasive therapy with an often age- and disease-related increased anesthesia risk. In addition, there is a danger of removing healthy thyroid tissue as well as injuring the adjacent parathyroid glands and delicate nerve structures during surgery.
5. Radioiodine therapy:
The international gold standard in the management of feline as well as human
hyperthyroidism. Radioactively labeled iodine is administered to the patient, accumulates exclusively in the neoplastic, overactive thyroid tissue, and destroys the tumor cells. Because the effective beta radiation penetrates only 2mm into the tissue, only hyperactive thyroid tissue is destroyed.
Normal thyroid tissue is inactive at this time and therefore does not take up radioactive iodine.
The additional gamma radiation from the radionuclide causes radiation exposure to the environment.
For this reason, patients are hospitalized until a legally defined safety threshold is reached to protect their human family. For the cat itself, the radiation is comparable to a CT scan.
In veterinary medicine, a maximum dose of 185 MBq is used for radiation-protection reasons; this cures 95 % of patients, while 5 % require a second treatment to achieve a cure.
In rare cases, hypothyroidism can occur after radioiodine therapy.
Regular follow-up examinations are therefore recommended for early detection so that appropriate treatment can be initiated.
General note on the kidneys:
Special attention must always be paid to the kidneys in cats.
Because the increased filtration rate associated with hyperthyroidism can mask pre-existing kidney dysfunction, removal of the thyroid gland or radioiodine therapy may lead to a rise in kidney values afterwards.
For this reason, we carry out careful preliminary examinations.
