Changes in thyroid and renal function after bilateral thyroidectomy in cats

Abstract

Background Iatrogenic hypothyroidism might worsen the prognosis of cats with azotemic CKD after thyroidectomy. Varying thyroxine concentrations influence utility of creatinine in assessing renal function. Symmetric dimethylarginine (SDMA) has limited studies in cats with changing thyroid status. Objectives Thyroid status is stable 6 months post‐thyroidectomy. Symmetric dimethylarginine and creatinine are linearly associated without influence from total thyroxine concentration (tT4). Animals Electronic records of 2 first opinion practices were searched using the term “thyroidectomy” to include 81 client‐owned cats that had undergone bilateral thyroidectomy. Methods Retrospective cross‐sectional study assessing thyroid hormone concentrations of 68 cats within 6 months of surgery. A longitudinal study of thyroid status in 23 cats with >18 months follow‐up post‐thyroidectomy. A generalized estimating equation assessed the associations of bodyweight, tT4 and creatinine concentrations on SDMA concentration. Results Sixty‐eight cats had follow‐up within 6 months. Fifteen cats (22%) had persistent, or recurrent, hyperthyroidism and 33 cats (49%) were hypothyroid. Twenty‐three of the euthyroid/hypothyroid cats had long‐term follow‐up (595‐1955 days); 4 cats (17%) remained hypothyroid, 19 cats (83%) were euthyroid (often transiently), and 9 of 23 cats (44%) developed recurrent hyperthyroidism. Symmetric dimethylarginine and creatinine were linearly associated, but hyperthyroid cats had higher SDMA concentrations, relative to creatinine (P = .003). Conclusions and Clinical Importance Cats have changes in thyroid function for years after bilateral thyroidectomy, with a high incidence of recurrent hyperthyroidism. Both SDMA and creatinine are affected by thyroxine concentrations, and the effect is greater in hyperthyroid cats.

A retrospective evaluation of the relationship between symmetric dimethylarginine, creatinine and body weight in hyperthyroid cats

“…By contrast, SDMA concentrations did not significantly decrease with increasing concentrations of TT4 (Fig 2B), suggesting that SDMA is more resistant to the effect of reduced muscle mass and increased GFR associated with increased TT4 concentrations than is CREA. A recent study on thyroidectomized hyperthyroid cats suggested a weak correlation between SDMA, bodyweight and TT4 levels [28]. In contrast, when pre-treatment cats were compared to control cats in the present study population, only pre-treatment cats had a significantly lower SDMA, and at no other time point was the SDMA concentration lower than the control population concentrations.…”

Treatment failure in hyperthyroid cats after radioiodine (I‐131) injection

“…However, recurrence of hyperthyroidism is common in cats having thyroidectomy performed in first‐opinion practice, even if cats are initially hypothyroid after the surgery. 15 The presence of ectopic hyperfunctional thyroid tissue is reported in 3.9%‐23% of cats undergoing thyroid scintigraphy indicating that surgical management would not be curative in these cases. 16 , 17 One argument in favor of surgical treatment is to obtain tissue for histopathological examination where thyroid carcinoma is suspected.…”

“…This treatment might have been chosen because it could be performed at lower cost than RAI therapy. However, recurrence of hyperthyroidism is common in cats having thyroidectomy performed in first‐opinion practice, even if cats are initially hypothyroid after the surgery 15 . The presence of ectopic hyperfunctional thyroid tissue is reported in 3.9%‐23% of cats undergoing thyroid scintigraphy indicating that surgical management would not be curative in these cases 16,17 .…”

The new age of renal biomarkers: does SDMAsolve all of our problems?

“…In a recent study examining changes in thyroid and renal function following bilateral thyroidectomy, SDMA was relatively higher in hyperthyroid cats compared to euthyroid or hypothyroid cats in a group of 68 client‐owned cats (Covey et al . 2019). Of interest, this study reported a high number of discordant results.…”