Results revealed hypothyroid dogs to have significantly higher basal GH concentrations than dogs with NTI. After TRH stimulation, GH levels increased significantly in hypothyroid dogs, whereas levels in dogs with NTI did not. Using a TRH-stimulated GH concentration cutoff of 1.95 µg/L at 30 minutes or 1.75 µg/L at 45 minutes allowed correct classification of all dogs. Basal TSH concentrations did not differ between groups; however, levels in dogs with NTI increased significantly in response to TRH stimulation, whereas levels in hypothyroid dogs did not. A cutoff at a 57% increase in TSH levels from baseline at 45 minutes poststimulation was used, and all dogs were correctly classified. In addition, the GH:TSH ratio was increased in hypothyroid dogs following TRH stimulation but decreased in dogs with NTI. A GH:TSH ratio cutoff of >4.6 allowed accurate classification of all hypothyroid patients.
No adverse events were noted following a TRH dose of 0.01 mg/kg in the current study, but higher doses (typically ≥0.1 mg/kg) have been associated with cholinergic side effects (eg, vomiting, hypersalivation).9