Cause
Feline hyperthyroidism is usually caused by benign adenomatous hyperplasia
(BAH) of the thyroid glands. Usually this affects both glands although
the cause is unknown. BAH causes uncontrolled excessive production of the
thyroid hormones, mainly thyroxine (T4).
Signalment and Clinical Signs
The disease affects elderly cats, usually over 10 years. It is rare before
seven years. All breeds of cat can be affected but it is rare in Siamese/Himalayan
breeds. Clinical abnormalities include weight loss, vomiting, diarrhoea,
polyuria, polydipsia, polyphagia, hyperactivity, unkempt coat, and a "starey"
expression. As awareness of the disease has increased earlier cases with
less marked clinical signs are now more frequently recognised.
Diagnostic Tests
Routine laboratory findings include increased liver enzymes (ALT and
ALKP in approximately 80% of cases), sometimes a mild-moderate azotemia,
inappropriately "normal" lymphocyte count or actual lymphocytosis
and sometimes a mild eosinophilia. Confirmation requires circulating total
T4 estimation. At Axiom, total T4 results > 60nmol/L are considered supportive.
Results between 40-60 nmol/L are equivocal. Options in equivocal cases include:
Wait and retest total T4 after a further 4-6 weeks
Free T4 (by dialysis) measurement
T3 suppression test
Please call the laboratory for further advice on handling equivocal cases.
Note that it is important to interpret the total T4 in the light of the
clinical signs. For example a severely cachectic cat with vomiting and PUPD
would usually be expected to have a low-normal or subnormal total T4 concentration.
An equivocal total T4 result in this situation may therefore increase the
index of suspicion for early hyperthyroidism.
Treatment
131I
treatment is the preferred option in cases that are otherwise healthy, since
the treatment is safe and effective, and usually curative. However this
is not an appropriate treatment for cats with concurrent disease (e.g.
congestive cardiac failure) and is limited due to the costs and
availability of appropriately licensed centres in the UK.
Surgical
thyroidectomy (often preceded by a period of medical stabilisation)
is frequently curative although associated with the risk of post-operative
hypocalcaemia (usually 1-4 days post op) due to inadvertent parathyroid
gland damage/removal. Bilateral thyroidectomies are usually preferred since
in most cats both glands are affected (even if grossly normal at
the time of surgery) but this obviously increases the risk of hypocalcaemia.
Medical
management is usually achieved with carbimazole (Neomercazole). The
starting dose is 5mg TID given at 8 hour intervals. Once thyroidal suppression
has been achieved (T4 less than 20nmol/L) the dose should be reduced
to 5mg BID permanently. Very few cats are well controlled on SID treatment.
Occasional adverse reactions to carbimazole are recognised and include agranulocytopenia,
thrombocytopenia, or rarely severe potentially fatal hepatopathies.
Therapeutic Monitoring
Total T4 concentrations less than 20nmol/L (irrespective of the treatment
used) is ideal in treated cats. Values more than 35 nmol/L should prompt
consideration of possible early recurrence. "Low" total T4 values
after therapy are rarely clinically significant and such cats should not
be medicated with thyroid hormone replacement therapy as this delays recovery
and regrowth of ectopic thyroid tissue. |