• Review classic signs of hyperthyroidism and contrast them with the typical presentation currently
• Discuss theories and evidence for these theories of the increasing incidence of hyperthyroidism
• Identify best testing strategies and when to use them
• Outline advantages and disadvantages of surgical and non-surgical therapies
• Veterinarians are in a position to diagnose early hyperthyroidism before the damage has been done
• Total T4 (TT4) is the best screening test, but a more complete feline panel or nuclear scintigraphy may be necessary if
TT4 is non-diagnostic
• Euthyroid sick syndrome is possible in cats and must be accounted for in your assessment
• I-131 is becoming more available and is the ideal permanent therapy for cats
• Methimazole (tapezole) can be used topically in difficult-to-pill patients
When hyperthyroidism was first reported in cats as a disease entity approximately 25 years ago, the majority of cases were
advanced. The cats were thin, aggressive, polyuric, polydipsic, polyphagic and had large palpable goiters. As cats have moved
out of barns where they served as 'mousers' and into peoples' homes as beloved family members, owners are expecting veterinarians
to provide excellent routine health care for their cats. Routine wellness exams give veterinarians an opportunity to diagnose
hyperthyroidism before many of the classic clinical signs occur. Diagnosis can be confounded by the multitude of concurrent
illnesses that can affect the aging cat. You may now be confronted with the fat, happy cat that has only recently begun urinating
more frequently in the litter box and has a trivial thyroid slip on physical examination. Yet this patient may be the ideal
candidate for anti-hyperthyroid therapy such as I-131.
In addition to a thorough physical exam, all geriatric cats (8 year or older) should have annual blood work performed. A minimum
database in the older cat includes complete blood count, biochemistry panel, urinalysis and a total T4. If the cat is an indoor/outdoor
cat or lives in a household where new cats are being introduced, FeLV/FIV should be tested as well. Illnesses such as diabetes
mellitus, renal disease, anemia, hypercalcemia and gastrointestinal disease are a few of the diseases/abnormalities which
can appear later in life and which may falsely lower the total T4 in a cat that actually has hyperthyroidism. Liver enzymes
may indicate concurrent hepatic disease or simply reflect the cat's hyperthyroid state, especially if only ALT is mildly elevated.
Ruling In (or Out) Hyperthyroidism
Total T4 remains the best, most inexpensive screening test to detect hyperthyroidism in cats. The time to consider additional
thyroid measurements, such as free T4 by equilibrium dialysis, are in those cases in which hyperthyroidism is suspected based
on clinical signs, but where total T4 is within normal limits. The gold standard for the diagnosis of hyperthyroidism is nuclear
scintigraphy. The number of facilities certified to provide nuclear scintigraphy and other radioactive testing/therapy are
In all cases of hyperthyroidism, treatment is directed at achieving a euthyroid status (T4 < 2.0) while maintaining reasonable
renal function. In cases in which renal insufficiency is severe once the cat becomes euthyroid, a permanent surgical or chemotherapeutic
solution is not recommended. Therefore, a "tapezole challenge" is essential for any cat whose owner is interested in pursuing
surgery or I-131 therapy. For owners who prefer medical therapy, or whose pets are ineligible for permanent therapy, options
include methimazole (oral), methimazole (transdermal formulations) and carbimazole. I-131 therapy is considered the ideal
therapy for those cats that maintain kidney function at normal thyroid levels. Advantages of I-131 therapy include permanent
therapy, minimally invasive technique, targets primarily the abnormal thyroid tissue, and almost no post-therapy complications.
I-131 therapy is expensive (although the expense is comparable to surgery with intense monitoring for post-surgical hypocalcemia),
requires isolation of the cat for up to 2 weeks after injection and may not be available to all general practitioners.
Topical therapy with methimazole can be an option for owners who are unable to elect a permanent therapy for their cat and
who have difficulty medicating their pet. Studies have shown that it is possible to achieve therapeutic levels of methimazole
in the bloodstream with transdermal formulations. Disadvantages include the greasy nature of the formulations, inconsistent
application and the usual problems of compliance with life-long therapy.
Hyperthyroidism is increasingly seen in the feline population, due in part to the increasing life-span and excellent veterinary
care these cats receive. Identifying and treating hyperthyroidism early requires testing for and recognizing more subtle signs
of the disease. Total T4 is the preferred screening test, but early disease may be identified with further assessment of free
T4 and/or nuclear scintigraphy. Non-invasive, permanent therapy, such as I-131 is becoming more available and is considered
the treatment of choice. Medical therapy is still a viable option, but can now be given transdermally in those patients that
do not tolerate twice-daily oral administration.
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