Small Animal Clinical Pharmacology - CYF MEDICAL DISTRIBUTION

CHAPTER 20 DRUGS USED IN THE MANAGEMENT OF THYROID AND PARATHYROID DISEASE
In general, the higher the pretreatment T 4 , the longer
it takes to achieve euthyroidism although in many cats
‘biochemical euthyroidism’ is achieved within days.
Obvious regression of clinical signs lags behind the
reduction in T 4 concentration but is generally apparent
after 2–3 weeks. Consequently, monitoring should begin
roughly 2 weeks after starting medication. If a circulating
total T 4 concentration is below or within the low
end of the reference range, the dose can be decreased by
2.5–5 mg to the final maintenance dose, or a surgical
thyroidectomy performed. If thyroid hormone concentrations
remain high, the duration of therapy is increased
or the dosage is altered in 2.5–5 mg increments. The cat
should be checked 2–3 weeks after each dose adjustment
and thereafter, every 3–6 months.
Despite marked suppression of circulating total T 4
concentration, clinical signs of hypothyroidism do not
develop. This is presumably because corresponding
circulating T 3 concentrations, the more metabolically
active hormone, remain within the reference range, possibly
through increased extra- or intrathyroidal production
of T 3. The latter may arise from intrathyroidal
iodine deficiency or increased TSH production induced
by antithyroid medication. In addition, circulating free
T 4 concentration, the active portion of total T 4 , appears
to remain somewhat higher than the total hormone
concentration during thiamazole therapy, suggesting a
potential shift in the binding affinity of circulating
proteins.
Medication should be stopped if serious adverse reactions
occur and an alternative form of therapy should
be sought.
Transdermal administration
Thiamazole can be incorporated into a pluronic lecithin
organogel (PLO) for transdermal application. This route
of administration may be of particular relevance for
fractious or inappetant cats or those with concurrent
gastrointestinal disease or drug-induced vomiting. The
bioavailability of thiamazole administered by this route
in healthy cats is variable but generally poor. However,
administration of 2.5 mg twice daily in hyperthyroid
cats has been shown to be effective although the time
to achieve euthyroidism in the majority of cats is longer
(approximately 4 weeks) than after oral administration.
Interestingly, cats treated with oral thiamazole have a
higher incidence of gastrointestinal adverse effects compared
to cats treated with transdermal thiamazole.
Adverse effects
Thiamazole and carbimazole are safer and better tolerated
than propylthioruracil although adverse reactions
certainly can occur (Table 20.1).
Mild adverse effects occur in approximately 10–15%
of cats and include anorexia, lethargy and vomiting.
Table 20.1 Adverse reactions associated with
thiamazole therapy
Reaction
Anorexia 10
Vomiting 10
Lethargy 10
Excoriations (facial) 2
Hepatopathy 1.5
Thrombocytopenia 2.5
Agranulocytosis 1.5
Leukopenia 4
Eosinophilia 10
Lymphocytosis 7
Positive ANA titer 50
Positive Coombs’ test 1.5
Myasthenia gravis