| We reviewed
the medical records of 494 cats with a variety of nonthyroidal diseases
in which serum thyroxine (T4) concentration was determined as part of
diagnostic evaluation. The cats were grouped by category of disease (ie,
renal disease, congestive heart failure, diabetes mellitus, focal neoplasia,
systemic neoplasia, hepatopathy, inflammatory bowel disease, inflammatory
pulmonary disease, miscellaneous diseases, or undiagnosed disease), degree
of illness (ie, mild, moderate, or severe), survival (ie, lived, died,
or euthanatized), and presence or absence of a palpable thyroid gland.
The mean (+/- SD) serum T4 concentrations in all 10 groups of cats, which
ranged from 10.5 +/- 11.1 nmol/L in cats with diabetes mellitus to 18.7
+/- 7.8 nmol/L in cats with focal neoplasia, were significantly (P less
than 0.001) lower than those of normal cats (27.0 +/- 10.4 nmol/L). The
number of ill cats with low serum T4 concentrations (less than 10 nmol/L)
was highest in the cats with diabetes mellitus (59%), hepatopathy (54%),
renal failure (48%), and systemic neoplasia (41%). When the serum T4 concentrations
in cats with mild, moderate, and severe illness were compared, mean concentrations
were progressively lower (21.3 +/- 6.8, 14.8 +/- 8.1, and 6.5 +/- 5.8
nmol/L, respectively) as degree of illness increased. Severity of illness
had a more significant (P less than 0.001) effect in lowering serum T4
concentrations than did disease category. Mean serum T4 concentrations
in the cats that died (7.8 +/- 9.8 nmol/L) or were euthanatized (10.0
+/- 7.0 nmol/L) were also significantly (P less than 0.001) lower than
those of cats that survived (15.2 +/- 8.8 nmol/L). Of the 182 cats with
low serum T4 concentrations, 74 (41%) died or were euthanatized. The 63
cats with palpable thyroid nodules had significantly (P < 0.001) higher
mean serum T4 concentrations (21.7 + 10.4 nmol/L) than the cats in which
a thyroid nodule was not palpated (12.7 + 8.1 nmol/L). Adenomatous hyperplasia
of the thyroid gland was confirmed at necropsy in 2 cats with a palpable
thyroid gland, and 4 cats that survived have subsequently developed overt
hyperthyroidism. This suggests that, in some cats with early or mild hyperthyroidism,
concomitant nonthyroidal disease may suppress serum T4 concentrations
into the normal range. |