Similar results were found in another study of 22 cats treated with radioiodine alone.¹⁴ Serum T₄, serum creatinine, and BUN concentrations and urine specific gravity were measured before treatment and six and 30 days after treatment. The glomerular filtration rate (GFR) was measured by using nuclear medicine imaging techniques for all 22 cats before treatment and six days after treatment for 21 of the cats. Significant decreases in serum T₄ concentrations occurred six days after treatment, but the changes in GFR and serum creatinine and BUN concentrations were not significant. Thirty days after treatment, significant increases in BUN and serum creatinine concentrations occurred as well as further significant decreases in serum T₄ concentrations. Nine cats were in renal failure before treatment, and 13 cats were in renal failure 30 days after treatment. These 13 cats included eight of the nine cats in renal failure before treatment (one cat was no longer in renal failure after treatment) and five cats not previously in renal failure. (Renal failure was defined as a BUN concentration greater than 30 mg/dl or a serum creatinine concentration greater than 1.8 mg/dl with a concurrent urine specific gravity of less than 1.035.) Evaluations after 30 days for nine of these 13 cats indicated that all remained in renal failure. The authors analyzed the cats' pretreatment GFR measurements as a predictor of posttreatment renal failure and derived a value of 2.25 ml/kg/min as a point of maximum sensitivity (100%) and specificity (78%). Fifteen of 22 cats had pretreatment GFR measurements below 2.25 ml/kg/min. These 15 cats included all nine cats in renal failure and five cats with normal renal clinicopathologic test results before treatment. Thirty days after treatment, 13 of these 15 cats were in renal failure. The two cats that were not in renal failure had persistently increased serum T₄ concentrations. Seven of the 22 cats had pretreatment GFR measurements greater than 2.25 ml/kg/min, and none of these seven cats was in renal failure 30 days after treatment. The authors concluded that significantly decreased renal function occurs after treating hyperthyroidism, and this decrease is clinically important in cats with renal disease. Measuring GFR before treating hyperthyroidism is valuable in detecting subclinical renal disease and in predicting which cats may have clinically important reductions in renal function.

In another study, researchers examined the effects of methimazole therapy on renal function in cats with naturally occurring hyperthyroidism.¹⁵ Twelve hyperthyroid cats and 10 clinically normal (control) cats were studied. Renal function was assessed with a single-injection plasma iohexol clearance test (a measurement of GFR), along with routine serum chemistry profiles and urinalyses. After the initial evaluation, all hyperthyroid cats were treated with methimazole at a starting dose of 5 mg given orally every 12 hours. Dose adjustments were made until euthyroidism was achieved. The mean pretreatment GFR, as estimated by the plasma iohexol clearance test, was significantly higher for the hyperthyroid cats than for the control cats. Control of hyperthyroidism significantly decreased the GFR when compared with pretreatment values. No significant difference was noted between the mean GFR for the posttreatment hyperthyroid cats and the mean GFR for the control cats. In the hyperthyroid group, the mean increases in BUN and serum creatinine concentrations and the mean decrease in urine specific gravity after treatment were not statistically significant when compared with pretreatment values. Two of the 12 hyperthyroid cats developed abnormally high serum creatinine concentrations after treatment. Both of these cats exhibited isosthenuria on pretreatment urinalyses and had posttreatment GFR values of less than 1 ml/kg/min. The authors thought that posttreatment reductions in GFR most likely unmasked latent renal dysfunction in these two cats. The authors concluded that when treating hyperthyroid cats suspected to have renal dysfunction, it is prudent to conduct a trial course of meth imazole with follow-up serum chemistry profiles and urinalyses before selecting a nonreversible therapy (e.g. thyroidectomy, radioactive iodine).

The effects of hyperthyroidism on renal function were also evaluated in a group of 10 normal adult cats in which hyperthyroidism was induced with exogenous administration of T₄.¹⁶ Baseline serum T₄ measurements, complete blood counts, serum chemistry profiles, urinalyses, and nuclear medicine assessments of GFR and effective renal plasma and renal blood flow were performed. The cats were then given T₄ (50 B5g/kg subcutaneously) daily for 30 days to induce hyperthyroidism. The clinicopathologic tests and nuclear medicine studies were repeated 30 days later, and the results revealed significant increases in T₄ concentrations, GFR, and effective renal blood flow and significant decreases in serum creatinine and BUN concentrations.