Hyperthyroid cats have elevated parathyroid hormone (PTH) concentrations and suppressed fibroblast growth factor-23 (FGF-23) concentrations, both of which normalise following treatment of hyperthyroidism. PTH, FGF-23, and thyroid hormone can influence the renal reabsorption of calcium and phosphate. The aim of the present study was to assess the influence of hyperthyroidism on renal tubular function in cats indirectly, by comparing the FE of electrolytes in hyperthyroid cats with and without chronic kidney disease (CKD), which could also alter the FE of electrolytes, before and after treatment of hyperthyroidism.
Hyperthyroid cats were treated for hyperthyroidism and monitored for the development of azotaemia over a 240-day follow up period. Blood and urine samples were collected at baseline and following treatment and the FE of calcium and phosphate calculated. The Wilcoxon signed rank test was used to compare FE before and after treatment. Data are presented as median (25th, 75th percentile).
Forty-two hyperthyroid cats (27 non-azotaemic and 15 azotaemic) were included in the study. In the non-azotaemic cats, there was an increase in the FE of calcium (0.10 (0.07, 0.15) vs 0.14 (0.10, 0.29) %; P=0.001) and phosphate following treatment (37.2 (19.8, 42.3) vs 51.9 (34.8, 63.0)%; P=0.001). Following treatment of azotaemic cats, there was an increase in the FE of calcium (0.24 (0.11, 0.49) vs 0.77 (0.19, 1.31) %; P=0.005), however the FE of phosphate did not increase significantly (48.1 (37.7, 58.9) vs 61.2 (47.6, 80.9) %; P=0.156).
FE of calcium and phosphate increased following treatment of hyperthyroidism, however the non-significant increase in FE of phosphate following treatment of azotaemic cats might indicate that the maximum level of phosphate excretion was reached in some animals.