Use of basal and TRH-stimulated plasma growth hormone concentrations to differentiate between primary hypothyroidism and nonthyroidal illness in dogs

BACKGROUND: A 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). HYPOTHESIS: Basal and TRH-induced plasma GH concentrations can be used to distinguish hypothyroid dogs from NTI dogs. ANIMALS: Twenty-one dogs with signs consistent with hypothyroidism, a low plasma TT4 concentration, and a plasma TSH concentration within reference interval. METHODS: Case 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. RESULTS: Eleven of the dogs were classified as hypothyroid and 10 as having NTI. Basal plasma GH concentration in the hypothyroid dogs (3.2 mug/l; range, 2.0 to 12.5 mug/l) was significantly higher (p