Objective: To determine whether measuring change in serum thyroid-stimulating hormone (TSH) concentration in response to thyrotropin-releasing hormone (TRH) administration can be used as a test of thyroid function in dogs suspected of having hypothyroidism.
Design: Case-cohort study.
Animals: 13 healthy dogs, 20 hypothyroid dogs, and 18 euthyroid dogs with concurrent diseases.
Procedure: Blood samples were collected before and 30 minutes after TRH administration, and serum TSH concentration was measured. The 13 healthy dogs were used to establish a reference range for change in TSH concentration after TRH administration. The remaining 38 dogs were categorized as hypothyroid or euthyroid on the basis of baseline total thyroxine (T4) and TSH concentrations, T4 concentration 4 hours after TRH administration, and clinical response to administration of sodium levothyroxine.
Results: Median baseline TSH concentration was 0.25 ng/ml (range, 0.03 to 0.44 ng/ml) in healthy dogs, 0.93 ng/ml (0.21 to 3.5 ng/ml) in hypothyroid dogs, and 0.21 ng/ml (0.03 to 0.63 ng/ml) in euthyroid dogs with concurrent diseases. Median percentage change in TSH concentration after TRH administration was 207% (range, 25 to 2,200%) in healthy dogs, 24% (-21 to 134%) in hypothyroid dogs, and 167% (69 to 1,800%) in euthyroid dogs with concurrent diseases. Overall accuracy of using the TRH-induced change in TSH concentration to identify hypothyroid dogs was 90%.
Clinical implications: Although percentage change in TSH concentration in response to TRH administration can be used to differentiate euthyroid from hypothyroid dogs, the test has little advantage over measurement of baseline TSH and total or free T4 concentration.