Long-Term Changes in Glomerular Filtration Rate in Hyperthyroid Cats Following Treatment with Iodine-131

Slater L., Neiger R., Haller M., et al.

Conference Proceedings, (2003). American College of Veterinary Internal Medicine: p.742


It is generally believed that there is a decline in renal function following treatment of hyperthyroidism in cats, but no long-term assessment of glomerular filtration rate (GFR) has been performed. Our aim was to assess the effect of iodine-131 treatment on immediate and long-term renal function in cats with hyperthyroidism. Thirteen cats finished the study so far. Serum biochemistry profile, total T4 (TT4), urine analysis, non-invasive blood pressure (NIBP) by Doppler method and GFR were measured pre-treatment and 1 and 6 months post treatment. Serum biochemistry profile, TT4 and NIBP were also measured 3 months post treatment. A single injection inulin clearance test and a two compartmental model were used to calculate GFR. The data was analysed using repeated measures ANOVA assuming a significance level of P < 0.05

TT4 decreased from mean 149 +/- SD 55 nmol/l to below reference range (RR) (20–65 nmol/l) in all but four cats for the entire study period. In one cat TT4 decreased after 3 months below the RR, in two cats TT4 decreased below the RR but increased into the RR at 6 months while in 1 cat TT4 decreased into the RR throughout the follow-up.

Seven cats had a pre-treatment GFR within the RR (2.07–3.69 ml/min/kg), 5 had increased GFR (range 4.26–6.14) and 2 had decreased GFR (range 0.9– 1.88). One of the latter cats died after 3 months from renal failure. Overall, there was a significant drop of GFR after 1 months but no further decline after 6 months. In the two cats with normalisation of TT4 there was a marked concurrent increase of GFR, a finding not seen in cats where TT4 remained below the RR. In the surviving cat with low initial GFR, TT4 dropped to the low end of the RR and GFR did not change over time. There was no significant difference in NIBP (p = 0.24) over time.

In conclusion, most hyperthyroid cats treated with iodine-131 showed a significant decline in GFR within one month following treatment, however, this decline did not continue. Results also indicated that a subnormal TT4 may contribute to this reduction in GFR as normalisation of TT4 resulted in marked increase in GFR. Therefore, a dose of iodine-131 should be used, which does not render TT4 below the normal reference range. If the same is true for medical or surgical management of feline hyperthyroidism needs to be assessed.