BackgroundA low plasma total thyroxine (TT4) concentration in combination with a plasma TSH concentration within reference range does not distinguish between hypothyroidism and nonthyroidal illness (NTI) in dogs. Hypothyroidism is associated with TSH‐releasing hormone (TRH)‐induced increased release of growth hormone (GH).HypothesisBasal and TRH‐induced plasma GH concentrations can be used to distinguish hypothyroid dogs from NTI dogs.AnimalsTwenty‐one dogs with signs consistent with hypothyroidism, a low plasma TT4 concentration, and a plasma TSH concentration within reference interval.MethodsCase control study. Thyroid scintigraphy was performed to classify dogs as having hypothyroidism or NTI. All dogs underwent a TRH stimulation test with measurement of plasma concentrations of GH and TSH before and 30 and 45 minutes after IV administration of TRH.ResultsEleven of the dogs were classified as hypothyroid and 10 as having NTI. Basal plasma GH concentration in the hypothyroid dogs (3.2 μg/l; range, 2.0 to 12.5 μg/l) was significantly higher (p<0.001) than that in the NTI dogs (.73 μg/l; range, .45 to 2.3 μg/l), with minimal overlap, and increased (p=.009) after TRH administration in hypothyroid dogs, whereas it did not change in NTI dogs. At T=45, plasma GH concentrations in hypothyroid dogs and NTI dogs did not overlap. The plasma TSH concentration did not change significantly after TRH administration in hypothyroid dogs, whereas it increased (p<.001) in NTI dogs. At T=45, there was no overlap in percentage TSH increase from baseline between hypothyroid dogs.Conclusions and Clinical ImportanceMeasurement of basal plasma GH concentration and concentrations of GH and TSH after TRH stimulation can distinguish between hypothyroidism and NTI in dogs.