Prevalence of feline hyperthyroidism in a laboratory-based sample of 27,888 cats in Spain

The objective of the study was to investigate the prevalence of hyperthyroidism in Spain, including its geographical distribution and prevalence across different age groups.

A cross-sectional study was conducted. Samples submitted to a reference laboratory to evaluate serum total thyroxine concentration (TT4) during a 3-year period were evaluated (n = 27,888). A cat was considered hyperthyroid if the TT4 concentration was greater than 60.4 nmol/l (4.7 μg/dl). Hyperthyroid cats were classified based on their TT4 concentration at diagnosis as mildly (TT4 60.4-124.8 nmol/l [4.7-9.7 μg/dl]), moderately (TT4 124.8-249.68 nmol/l [9.7-19.4 μg/dl]) or severely (TT4 >249.6 nmol/l [>19.4 μg/dl]) hyperthyroid. The samples were also grouped by life stage, into young adults (1-6 years old), mature adults (7-10 years old) and seniors (>10 years old). The study included samples from 17 regions of Spain and geographical mapping was based on seven domains.

One thousand seven hundred and twenty-two out of 27,888 cats (6.2%, 95% confidence interval [CI] = 5.9-6.5) were classified as hyperthyroid. Within the hyperthyroid group, ages ranged from 2 to 25 years old, with a median of 14 years. Within the hyperthyroid population, 14/368 (3.8%), 27/368 (7.3%) and 327/368 (88.8%) were young adults, mature adults and seniors, respectively. The prevalence of hyperthyroidism in this laboratory-based sample in Spain was 2.7 (95% CI = 0.4-5.7) to 6.9% (95% CI = 6.4-7.4), depending on the geographical region. During the study period, 8.2% of cases that were initially non-hyperthyroid were later diagnosed with hyperthyroidism.

Prevalence of hyperthyroidism in Spain is 6.2% (95% CI = 5.9-6.5) overall and 7.9% (95% CI = 7.2-8.8) in cats older than 10 years. The distribution of the disease is heterogeneous between different geographical regions. Further large-scale prospective studies are required to determine the risk factors in each region and understand the reasons for this variation.