Restoration of Euthyroidism in Medically Treated Hyperthyroid Cats with Iatrogenic Hypothyroidism (IH) Improves Renal Function.

Williams T.L., Elliott J. and Syme H.

Conference Proceedings, (2012). American College of Veterinary Internal Medicine, New Orleans: p.752-754


IH is reported to increase the incidence of azotemia in hyperthyroid cats following treatment. IH is also associated with a decreased heart rate (HR), packed cell volume (PCV) and plasma alkaline phosphatase activity (ALP). Hypothyroidism reduces glomerular filtration rate (GFR) in other species, and treatment of hypothyroidism in dogs is reported to increase GFR. Therefore, it could be postulated that adjustment of anti-thyroid medication in cats with IH to achieve euthyroidism would also improve GFR, and hence reduce plasma creatinine concentration and the incidence of azotemia. This study aimed to compare renal function in cats with IH before and after dose adjustment of anti-thyroid medication (methimazole or carbimazole) to achieve euthyroidism.

Medically treated hyperthyroid cats with documented IH (plasma total thyroxine concentration (TT4) <10 nmol/l in combination with plasma cTSH >0.15 ng/ml) were prospectively recruited into the study between July 2009 and March 2011. Dose adjustments were made to anti-thyroid medication every four weeks until euthyroidism was restored (TT4 10-40 nmol/l). If restoration of euthyroidism was achieved more than eight months after documentation of IH, the cat was excluded from further analysis. Body weight (BW), body condition score (BCS), systolic blood pressure (SBP), PCV, HR and plasma concentra- tions of urea, creatinine, cholesterol, total calcium and plasma activities of alanine aminotransferase (ALT) and ALP were recorded at baseline and once euthyroidism was achieved. Continuous data are presented as median [25th, 75th percentile]. Clinicopathological parameters were compared at baseline and following restoration of euthyroidism using the Wilcoxon signed rank test. Statistical significance was defined as P<0.05.

Twelve cats were eligible for inclusion. Baseline TT4 and cTSH of the cats were <4.0 [<4.0, 6.1] nmol/l and 3.45 [0.48, 5.93] ng/ml respectively. At baseline, 9/12 cats were azotemic (plasma creatinine concentration >2 mg/dl). TT4 following dose adjustment was 24.9 [15.4, 26.6] nmol/l. Restoration of euthyroidism resulted in a significant decrease in plasma creatinine concentration (2.50 [1.92, 3.16] mg/dl vs. 2.07 [1.42, 2.66] mg/dl; P=0.005), and azotemia resolved in 3/9 cats. Restoration of eu- thyroidism also resulted in a significant increase in plasma ALP (20.8 [15.8, 30.8] U/l vs. 31.5 [23.2, 45.7] U/l; P=0.004) and HR (186 [174, 204] beats per minute (bpm) vs. 214 [182, 225] bpm; P=0.034), and a significant decrease in BW (3.91 [3.36, 4.15] kg vs. 3.70 [3.23, 4.16] kg; P=0.026). No significant changes in BCS (P=0.603), SBP (P=0.875), PCV (P=0.329), plasma concentra- tions of urea (P=0.239), cholesterol (P=0.530), total calcium (P=0.182) or plasma ALT activity (P=0.388) were observed fol- lowing restoration of euthyroidism.

Restoration of euthyroidism in cats with IH improves renal function, evaluated using plasma creatinine concentration, and decreases the incidence of azotemia, although changes in BW could influence plasma creatinine concentrations. Measurement of GFR in cats with IH before and after dose adjustment is warranted.