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 hyperthyroid 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 in cats 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.(ABSTRACT TRUNCATED AT 250 WORDS)