Use of the Thyrotropin Releasing Hormone Stimulation Test to Diagnose Mild Hyperthyroidism in Cats

We evaluated serum T₄ and T₃ concentrations before and after administration of thyrotropin releasing hormone (TRH) in 35 cats with mild to moderate hyperthyroidism. 15 cats with nonthyroidal disease, and 31 clinically normal cats. The TRH stimulation test was performed by collecting blood for serum T₄ and T₃ determinations before and 4 hours after IV administration of 0.1 mg/kg TRH. Mean basal serum thyroid hormone concentrations in hyperthy-roid cats were significantly (P < .05) higher than concentrations in normal cats and in those with nonthyroidal disease, but there was considerable overlap among the 3 groups. After administration of TRH, mean serum T₄ concentrations increased significantly in all groups of cats, whereas mean T₃ concentrations increased significantly in normal cats and in those with nonthyroidal disease, but not in cats with hyperthyroidism. The absolute difference between mean basal and TRH-stimulated serum concentrations of T₄ in cats with hyperthyroidism (10.7 nmol/L) was significantly lower than the difference in the cats with nonthyroidal disease (20.0 nmol/L) and in clinically normal cats (28.3 nmol/L), but there was considerable overlap in values among groups. The mean value for relative change in serum T₄ concentration after TRH was significantly lower incats with hyperthyroidism (18.9%) than in those with nonthyroidal disease (110.0%) and in clinically normal cats (130.2%). Serum T₄ concentrations increased by > 50% in all normal cats and cats with nonthyroidal disease, whereas only 4(11.4%) of the 35 hyperthyroid cats had an increase of > 50% after TRH administration. On the basis of canonical discriminate analysis, the mean discriminant function score was significantly higher in the hyperthyroid cats (D = 63.8) than in cats with nonthyroidal disease (D = 5.9) or clinically normal cats (D = 0.7). All cats having a discriminant function score > 30 were hyperthyroid, whereas all cats with a value < 20 were euthyroid. Adverse side effects associated with administration of TRH were common and included transient vomiting, salivation, tachypnea, and defecation. Results of this study indicate that the TRH stimulation test is a useful aid in the diagnosis of hyperthyroidism in cats when basal serum T₄ concentrations are high-normal or only slightly high. As a diagnostic test, the TRH stimulation test compares favorably with the T₃ suppression test but requires less time and is more convenient to perform.