| 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.
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. |