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Medical management
Methimazole and carbimazole are the two antithyroid drugs most frequently used for preoperative and long-term medical management of hyperthyroidism because of their consistent and potent effect in lowering thyroid hormone concentrations and relatively limited occurrence of serious adverse effects. Both are actively concentrated by the thyroid gland where they inhibit thyroid hormone production. Carbimazole is converted to methimazole and only methimazole accumulates in the thyroid gland. Methimazole is available as both human and veterinary licenced formulations (Felimazole® (1.25, 2.5 and 5 mg), Dechra Veterinary Products; Thiafeline® (2.5 and 5 mg), Animalcare; Thyronorm® (5 mg/mL
liquid), Norbrook). Carbimazole is also available as
a preparation for human use and a novel once daily controlled-release formulation (10 or 15 mg tablets) is licenced for cats in Europe (Vidalta® (10 and 15 mg), MSD Animal Health). Administration with food significantly enhances absorption. For long-term management, once euthyroidism has been achieved, the daily dosage is adjusted aiming for the lowest possible dose that effectively maintains control.Long-term monitoring of cats involves regular assessment of clinical signs and serum total T4 measurements every three to six months with reassessment of total T4 concentration 10 days to 3 weeks after each dose adjustment. Whether antithyroid medication is given once or twice daily has little impact on the timing of samples for monitoring purposes5. It is generally accepted that total T4 concentration should be maintained within the lower end of the reference interval6. In many cats, antithyroid drug therapy results
in serum total T4 concentrations below the reference interval. Although overt clinical signs of hypothyroidism rarely develop, and surgical risks are not increased, hypothyroidism should be avoided because if its detrimental effect on kidney function.Compliance
with oral medication can be problematic in fractious
or inappetant cats. Drug absorption is also potentially affected by concurrent gastrointestinal disease particularly for those cats that vomit. Methimazole
and carbimazole can be reformulated for transdermal application and appear equally as efficacious as oral preparations7. Recommendations for starting doses
and follow-up adjustments are similar for transdermal and oral administration. There are few commercially available transdermal products. Custom formulation increases expense of therapy and stability of the product is not guaranteed. There are health concerns regarding exposure of humans to methimazole.There is growing evidence that medical therapy may not be as efficacious in the long-term. Persistent hyperthyroidism, recurrent hyperthyroidism, increasing difficulty in controlling hyperthyroidism despite increasing drug dosages, and a yo-yo effect between hypothyroidism and hyperthyroidism appear to be common over time.
Continuing efficacy of antithyroid medication is highly dependent on good owner and cat compliance and this can be difficult to maintain over prolonged periods. Importantly medical management does not address the underlying cause of hyperthyroidism and over time the pathological changes in the thyroid gland progress and the prevalence of extreme goitre, multifocal lesions, intrathoracic thyroid masses and suspected malignant transformation increase8. Lack of efficacy long-term should be considered as a disadvantage of oral/ transdermal medication.
Restricted iodine diets
A restricted iodine diet has been marketed for optimising thyroid health (Hill’s y/d). Exclusively feeding this
diet has been shown to normalize thyroid hormone concentrations in some affected animals but usually only in mildly affected cases9. However, clinical signs may not fully reverse suggestive of persistent hyperthyroidism throughout treatment and the time to develop euthyroidism can be prolonged. There appear to be issues of compliance that may be related to palatability. In cats where other treatments are refused or not feasible it may be a useful option. Other food sources must not be available.
Surgical thyroidectomy
In practice, surgical thyroidectomy is often considered a treatment of choice particularly if radioactive iodine is unavailable. The majority (>70 %) of cats require bilateral thyroidectomy. If a unilateral thyroidectomy
is carried out future monitoring for recurrence of the condition is required. Routine bilateral thyroidectomy, whilst increasing the risk of post-operative complications, obviates the need for decision-making at the time of surgery. However, ectopic thyroid tissue should be considered if hyperthyroidism persists after routine thyroidectomy. A major concern is the development of postoperative hypocalcaemia that occurs within one
to five days of surgery. It is difficult to accurately assign
a risk for hypocalcaemia in a cat undergoing bilateral thyroidectomy as it is highly surgeon dependent. However, with experience, a low rate of postoperative hypocalcaemia (< 10 %) and recurrence (5 %) is expected. As for medical treatment regular postoperative monitoring is required to ensure that permanent hypothyroidism does not develop.
Assessment for hypothyroidism
Irrespective of treatment modality, hypothyroidism
is a potential sequel. Although no strict rules exist, suppressed total T4 values should prompt consideration of further investigation for hypothyroidism. Alternatively it is probably best practice to monitor cats through measurement of total T4 and TSH as it appears that TSH concentrations more accurately reflect reduced thyroid function10. Given that high TSH values are expected,
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