| The purpose
of this study was to develop a T3 suppression test to help in the diagnosis
of mild hyperthyroidism in cats. We evaluated the response in circulating
T4 concentrations to exogenous T3 (liothyronine) administration in 44
clinically normal cats, 77 cats with hyperthyroidism, and 22 cats with
nonthyroidal disease. The test was performed by first collecting blood
samples for basal serum T4 and T3 determinations, administering liothyronine
at an oral dosage of 25 micrograms three times daily for seven doses,
and, on the morning of the third day, again collecting serum samples for
T4 and T3 determinations 2 to 4 hours after the seventh dose of liothyronine.
The mean basal serum concentrations of T4 (53.1 nmol/L) and T3 (1.8 nmol/L)
were significantly higher in the cats with hyperthyroidism than in the
normal cats (T4 = 25.3 nmol/L, T3 = 1.3 nmol/L) and the cats with nonthyroidal
disease (T4 = 29.5 nmol/L, T3 = 1.4 nmol/L); however, there was a great
deal of overlap of basal values between the three groups of cats. Of the
77 cats with mild hyperthyroidism, 41 (53%) had serum T4 values and 55
(71%) had T3 values that were within the established normal ranges. After
administration of liothyronine, mean serum T4 concentrations fell much
more markedly in the normal cats and the cats with nonthyroidal disease
than in the hyperthyroid cats. The mean post-liothyronine serum T4 concentrations
in the hyperthyroid cats (48.5 nmol/L) were significantly higher (p <
0.001) than that of either the normal cats (9.5 nmol/L) or cats with nonthyroidal
disease (10.6 nmol/L). The mean percent decrease in T4 concentrations
after administration of liothyronine in the hyperthyroid cats (9.1%) was
also significantly (p < 0.001) lower than the decrease in either the normal
cats (63%) or cats with nonthyroidal disease (65%). The results of this
study indicate that the T3 suppression test is a useful aid in the diagnosis
of mild hyperthyroidism in cats when basal serum T4 concentrations are
high-normal or only slightly elevated. The T3 suppression test appears
to be of particular value in differentiating between hyperthyroid cats
and cats with nonthyroidal diseases that have basal serum T4 values in
the normal to high-normal range, since there was very little overlap in
the amount of T4 suppression between these cats. |