Iatrogenic Hypothyroidism (Ih) Contributes To The Development Of Azotemia In Hyperthyroid Cats.

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Williams TL, Elliott J, Syme HM.

in Conference Proceedings. American College of Veterinary Internal Medicine 2010;684.

IH is reported to occur in some cats following treatment of hyper- thyroidism (HTH). Hypothyroidism reduces GFR in other species, therefore IH could contribute to the development of azotemia in hyperthyroid cats with underlying mild chronic kidney disease. Our hypothesis was that the incidence of post treatment azotemia would be greater in cats with IH than euthyroid cats. Records from two London-based first opinion clinics between 1999 and 2009 were reviewed to identify cats diagnosed with HTH without pre-treatment azotemia. Baseline age and plasma creatinine concentration was recorded. HTH was treated using anti-thyroid medication alone or in combination with thyroidectomy. Cats were monitored for the development of azotemia over a six month follow up period, and at the final visit the following plasma biochemical and physical examination data were recorded; total thyroxine (TT4) concentration, cholesterol concentration, creatinine concentration, alanine aminotransferase and alkaline phosphatase activities, packed cell volume (PCV), heart rate and body weight. Only cats with a TT4 o 40 nmol/l at the end of the follow up period were el- igible for inclusion in the final analysis. Plasma TSH concentrations at the same time point were measured using the Immulite canine TSH assay. IH was defined as TT4 o 10 nmol/l in combination with TSH 4 0.15 ng/ml. Cats of uncertain thyroid status (TT4 10– 40 nmol/l and TSH 4 0.15 ng/ml) were excluded from the analysis. The Fisher’s Exact Test was used to compare the proportion of cats with azotemia between the IH and euthyroid groups. Continuous data are presented as median [25th, 75th percentile]. Clinicopatho- logical parameters were compared between the IH and euthyroid groups using the Mann Whitney U test. Statistical significance was defined as P o 0.05. Eighty-one cats were eligible for the study, of which 28 were classified with IH and 48 as euthyroid. Five cats were excluded as they were of uncertain thyroid status. No significant difference in age (P 5 0.063) or plasma creatinine concentration (P 5 0.963) at base- line was present between the two groups. Sixteen cats with IH (57%; 95% CI 39–75%) and fourteen euthyroid cats (29%; 95% CI 16– 42%) developed azotemia. The proportion of cats with azotemia in the IH group was significantly greater than the euthyroid group (P 5 0.028). Cats with IH had significantly lower plasma alkaline phosphatase activity (21 [14, 41] IU/L vs. 45 [30, 75] IU/L; P o 0.001), PCV (30 [27, 35]% vs. 36 [30, 40]%; P 5 0.004) and heart rate (180 [160, 192] bpm vs. 189 [171, 201] bpm; P 5 0.034) than euthyroid cats. Cats with IH also had significantly higher plasma creatinine concentrations (2.20 [1.86, 3.14] mg/dl vs. 1.85 [1.54, 2.18] mg/dl; P 5 0.01). No other parameters were significantly different between the two groups. The incidence of post treatment azotemia was significantly higher in cats with IH. Clinicopathological changes in cats with IH were consistent with those that occur in naturally occurring hypothyroidism in other species.