Effect of thyroxine supplementation on glomerular filtration rate in hypothyroid dogs

Glomerular filtration rate (GFR) is decreased in humans with hypothyroidism, but information about kidney function in dogs with hypothyroidism is lacking.

Hypothyroidism influences GFR in dogs. The objective of this study was to assess GFR in hypothyroid dogs before implementation of thyroxine supplementation and after re-establishing euthyroidism.

Fourteen hypothyroid dogs without abnormalities on renal ultrasound examination or urinalysis.

Blood pressure and GFR (measured by exogenous creatinine clearance) were measured before treatment (T0, n=14) and at 1 month (T1, n=14) and at 6 months (T6, n=11) after beginning levothyroxine supplementation therapy (20 microg/kg/d, PO). The response to therapy was monitored at T1 by measuring serum total thyroxine and thyroid stimulating hormone concentrations. If needed, levothyroxine dosage was adjusted and reassessed after 1 month. Statistical analysis was performed using a general linear model. Results are expressed as mean+/-standard deviation.

At T0, the average age of dogs in the study group was 6.3+/-1.4 years. Their average body weight decreased from 35+/-18 kg at T0 to 27+/-14 kg at T6 (P<.05). All dogs remained normotensive throughout the study. GFR increased significantly with levothyroxine supplementation; the corresponding results were 1.6+/-0.4 mL/min/kg at T0, 2.1+/-0.4 at T1, and 2.0+/-0.4 at T6 (P<.01).

GFR was <2 mL/min/kg in untreated hypothyroid dogs. Re-establishment of a euthyroid state increased GFR significantly.