| We measured
serum thyroxine (T4) and 3,3',5-triiodothyronine (T3) concentrations in
hyperthyroid cats (hourly for 10 hours in 14 cats, and daily for 15 days
in seven cats) to assess fluctuation in thyroid hormone levels. Over the
10-hour study period the coefficient of variation (CV) for serum T4 and
T3 concentrations ranged from 6.4-22.6% (mean = 12.0 +/- 4.8%) and from
9.6-33.1% (mean = 17.5 +/- 6.3%), respectively. During the 15-day study
period, CV for serum T4 ranged from 6.6-34.8% (mean = 18.4 +/- 9.3%),
while CV for serum T3 ranged from 7.8-31.0% (mean = 20.1 +/- 8.6%). These
CV values were significantly higher than the expected intra-assay CVs
(T4 assay, 5.1%; T3 assay, 7.7%). In addition, some of the cats with mild
hyperthyroidism showed one or more normal serum T4 and T3 values during
the course of the respective study periods. There was no specific time
during the 10-hour study period at which the cats consistently showed
peak serum T4 or T3 concentrations. These results suggest that serum thyroid
hormone concentrations are subject to a degree of fluctuation that exceeds
the usual assay variation, and that cats with mild hyperthyroidism can,
at a given time, exhibit normal serum T4 and T3 values. Therefore, a diagnosis
of feline hyperthyroidism should not be excluded on the basis of the finding
of a single normal serum T4 or T3 value in a cat with clinical signs and
physical examination findings consistent with the disease. |