Feline hyperthyroidism: current treatment options and the role of the veterinary nurse

Feline hyperthyroidism has become the most common endocrine disorder in older cats (mean 13 years of age, range 4 to 20 years of age (Nelson, 2007)) since the first case was described in 1979 (Peterson et al, 1979). The most recent epidemiological survey reported a prevalence of 21% of 508 cats 10 years of age or older in the greater Dublin area, Ireland (Gallagher and Mooney, 2013). Pedigree cats are under represented and there is no gender bias (Nelson, 2007). Feline hyperthyroidism is the result of over-production of thyroxine by one or more tumours of thyroid tissue. The normal feline thyroid gland is composed of two lobes located laterally on either side of the proximal trachea. Rarely, ectopic thyroid tissue can arise from the base of the tongue to the mediastinum (Patnaik et al, 2000). The loose attachments to the trachea allow enlarging adenomas or carcinomas to reach and enter the cranial thoracic inlet as a result of the effect of gravity. Most tumours are benign adenomas, with carcinomas accounting for fewer than 2% of cases (Peterson and Broome, 2014). However, a recent study of 2096 hyperthyroid cats referred for radioiodine treatment has suggested that this prevalence can reach 19.3% in cats treated with oral methimazole for 4 or more years (Peterson et al, 2015). The authors concluded that their study provides evidence that feline hyperthyroidism is a condition that progresses over time, with enlargement of tumours and transformation to malignancy, which cannot be arrested by medical management. The role of methimazole in this process, if any, requires further study. However, they acknowledged that their study population may not be representative of the whole population of cats managed with methimazole.