Objective—To evaluate response of euthyroid cats to
administration of recombinant human thyroid-stimulating
Animals—7 healthy cats.
Procedure—Each cat received each of 5 doses of
rhTSH (0, 0.025, 0.050, 0.100, and 0.200 mg), IV, at 1-week intervals. Serum concentration of total thyroxine
(TT4) and free thyroxine (fT4) was measured immediately
before each injection (time 0) and 2, 4, 6, and
8 hours after administration of each dose.
Results—Overall TT4 response did not differ significantly
among cats when administered doses were ≥
0.025 mg. Serum TT4 concentrations peaked 6 to 8
hours after administration for all doses ≥ 0.025 mg.
For all doses ≥ 0.025 mg, mean ± SEM TT4 concentration
at 0, 6, and 8 hours was 33.9 ± 1.7, 101.8 ± 5.9,
and 101.5 ± 5.7 nmol/L, respectively. For all doses ≥
0.025 mg, mean fT4 concentration at 0, 6, and 8 hours
was 38.7 ± 2.9, 104.5 ± 7.6, and 100.4 ± 8.0 pmol/L,
respectively. At 8 hours, the fT4 response to 0.025
and 0.050 mg was less than the response to 0.100
and 0.200 mg. Adverse reactions after rhTSH administration
were not detected.
Conclusions and Clinical Relevance—The TSH stimulation
test can be performed in cats by IV administration
of 0.025 to 0.200 mg of rhTSH and measurement
of serum TT4 concentrations at time of injection
and 6 or 8 hours later. Clinical validation of the TSH
stimulation test would facilitate development of additional
tests of thyroid gland function, such as a TSH
assay. (Am J Vet Res 2003;64:149–152)
Objective—To compare effects of short-term administration
of a soy diet with those of a soy-free diet on
serum thyroid hormone concentrations in healthy
Animals—18 healthy adult cats.
Procedure—Cats were randomly assigned to receive
either a soy or soy-free diet for 3 months each in a
crossover design. Assays included CBC, serum biochemical
profile, thyroid hormone analysis, and measurement
of urinary isoflavone concentrations.
Results—Genistein, a major soy isoflavone, was
identified in the urine of 10 of 18 cats prior to dietary
intervention. Compared with the soy-free diet, cats
that received the soy diet had significantly higher total
thyroxine (T4) and free T4 (fT4) concentrations, but
unchanged total triiodothyronine (T3) concentrations.
The T3/fT4 ratio was also significantly lower in cats
that received the soy diet. Although the magnitudes
of the increases were small (8% for T4 and 14% for
fT4), these changes resulted in an increased proportion
of cats (from 1/18 to 4/18) that had fT4 values
greater than the upper limit of the laboratory reference
range. There was no significant effect of diet on
any other measured parameter.
Conclusions and Clinical Relevance—Short-term
administration of dietary soy has a measurable
although modest effect on thyroid hormone homeostasis
in cats. Increase in T4 concentration relative to
T3 concentration may result from inhibition of 5'-iodothyronine deiodinase or enhanced T3 clearance.
Soy is a common dietary component that increases
serum T4 concentration in cats. ( Am J Vet Res 2004;