Serum thyroxine concentrations in clinically healthy pet guinea pigs (Cavia porcellus)

Background Guinea pigs (Cavia porcellus) are often presented as patients in veterinary practice. Nevertheless, only limited information is available about endocrine diseases or thyroxine reference values for the species. Objective The aim of this study was to determine serum thyroxine concentrations in a well-defined population of clinically healthy pet guinea pigs. Methods Between October 2007 and July 2008, serum samples were collected from 40 clinically healthy guinea pigs of different sexes, ages, and breeds that were presented to our clinic for a general health check or for castration. Pregnant females were excluded from the study. Thyroxine concentration was measured using a chemiluminescence test (Immulite 2000 Canine Total T4). Results Thyroxine concentrations ranged from 14.2 to 66.9 nmol/L (1.1-5.2 mu g/dL) with a median value of 27.0 nmol/L (2.1 mu g/dL). Females (n=16) had significantly (P=.039; Mann-Whitney U-test) lower thyroxine values than castrated males (n=8), whereas no differences were found between females and intact males (n=16) or between intact and castrated males. No significant correlation was found between thyroxine concentration and age. Conclusion This is the first report of serum thyroxine reference values for a well-defined population of healthy pet guinea pigs as measured by a chemiluminescence assay. The results were higher than those previously reported for this species and emphasize the importance of using appropriate reference intervals for the diagnosis of hyperthyroidism.