Functional hyperthyroidism as a result of thyroid neoplasia in guinea pigs (Cavia porcellus) has recently been documented and appears to be encountered in the clinical setting, but it is a poorly documented condition in these animals.1,2 The relative prevalence is 4.6% at 1 pathology facility, making thyroid neoplasia the second most commonly reported malignancy after lymphoma.1 However, the percentage of these nodules, which are actively secreting hormone or are active metabolically, is currently unknown. Similar to this observation, a German study3 found that hyperthyroidism is a relatively common clinical condition in guinea pigs. Typical clinical signs include hyperactivity, hyperesthesia, and polyphagia with weight loss. Most guinea pigs with hyperthyroidism have an elevated resting thyroxine concentration and a palpable neck mass. However, some animals have only marginally elevated thyroxine concentrations and a rare few have normal or low resting thyroxine concentrations; not all hyperthyroid guinea pigs have a palpable neck mass. Until very recently, there have been few validated reference values for thyroid hormone concentrations in guinea pigs, making interpreting results of a hormone profile problematic. Occasionally, guinea pigs are examined with typical clinical signs of hyperthyroidism or have no palpable neck tumor but are euthyroid on a hormone profile. These cases, in particular, are difficult to accurately diagnose as hyperthyroidism.
Normal thyroxine and triiodothyronine concentration reference ranges for guinea pigs were published in 1986.4 In addition, a recent study5 also established thyroxine concentrations for euthyroid guinea pigs. These concentrations ranged from 14.2 to 66.9 nmol/L (1.1 to 5.2 μg/dL), with a median of 27.0 nmol/L (2.1 μg/dL).5 Thyroxine concentration determined by radioimmunoassay requires a very small amount of plasma (25 μL). A TSH stimulation test can be an effective test for diagnosing some of these difficult-to-diagnose cases. If thyroxine concentration is evaluated for only a single sample, it is impossible to differentiate true hypothyroidism from euthyroid sick syndrome, in which animals with nonthyroidal illness can have substantial decreases in circulating thyroxine concentrations.
Hypothyroidism in animals can be congenital, acquired, or iatrogenic (as a result of treatment of hyperthyroidism). For dogs and cats, it has been customary to use bovine TSH for the TSH stimulation test; however, bovine TSH is no longer available as a pharmaceutical preparation. The hormone has been replaced with the notably more expensive rhTSH. Because of this development, new protocols for the TSH stimulation test in companion animals have been established for the use of rhTSH. Protocols have been described for dogs and cats, but we are not aware of a similar protocol for guinea pigs.6,7
The purpose of the study reported here was to evaluate the effects of rhTSH on plasma thyroxine concentrations in euthyroid guinea pigs to develop an antemortem method for evaluating thyroid function in guinea pigs. The specific hypotheses were that rhTSH can be safely and effectively used for TSH stimulation testing of guinea pigs, an increase in thyroxine concentration would be noted in animals given rhTSH, there would be no significant difference between the 3- and 4-hour sampling periods, and there would be no significant difference in thyroxine concentrations between sexes before or after the rhTSH stimulation test.
Recombinant human thyroid-stimulating hormone
Coat-a-Count, Siemens Healthcare Diagnostics, Los Angeles, Calif.
Thyrogen, Genzyme Corp, Cambridge, Mass.
SPSS, version 19.0, SPSS Inc, Chicago, Ill.
Ward C, University of Georgia, Athens, Ga: Personal communication, 2011.
Immulite 2000 Canine Total T4, Diagnostic Products Corp, Los Angeles, Calif.
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