Evaluation of recombinant human thyroid-stimulating hormone to test thyroid function in dogs suspected of having hypothyroidism

Objective-To evaluate the use of recombinant human (rh) thyroid-stimulating hormone (TSH) in dogs with suspected hypothyroidism. Animals-64 dogs with clinical signs of hypothyroidism Procedures-Dogs received rhTSH (75 mu g/dog, IV) at a dose independent of their body weight. Blood samples were taken before and 6 hours after rhTSH administration for determination of total serum thyroxine (T-4) concentration. Dogs were placed into 1 of 3 groups as follows: those with normal (ie, poststimulation values indicative of euthyroidism), unchanged (ie, poststimulation values indicative of hypothyroidism; no thyroid gland stimulation), or intermediate (ie, poststimulation values between unchanged and normal values) post-TSH T-4 concentrations. Serum canine TSH (cTSH) concentration was determined in prestimulation serum (ie, before TSH administration). Results-14, 35, and 15 dogs had unchanged, normal, and intermediate post-TSH T4 concentrations, respectively. Basal T4 and post-TSH T4 concentrations were significantly different among groups. On the basis of basal serum T4 and cTSH concentrations alone, 1 euthyroid (normal post-TSH T4, low basal T4, and high cTSH concentrations) and 1 hypothyroid dog (unchanged post-TSH T4 concentration and low to within reference range T4 and cTSH concentrations) would have been misinterpreted as hypothyroid and euthyroid, respectively. Nine of the 15 dogs with intermediate post-TSHT4 concentrations had received medication known to affect thyroid function prior to the test, and 2 of them had severe nonthyroidal disease. Conclusions and Clinical Relevance-The TSH-stimulation test with rhTSH is a valuable diagnostic tool to assess thyroid function in selected dogs in which a diagnosis of hypothyroidism cannot be based on basal T4 and cTSH concentrations alone.