Iatrogenic hypothyroidism can develop post-radioiodine treatment for feline hyperthyroidism; its incidence may be dose-dependent. Cats that develop iatrogenic hypothyroidism have a greater incidence of development of azotaemia than cats that remain euthyroid. Dogs with experimentally induced hypothyroidism have both decreased glomerular filtration rate (GFR) and endogenous creatinine production. This suggests that using creatinine as a marker of kidney function in hypothyroid cats could underestimate the incidence of development of kidney disease. This longitudinal study evaluated thyroid and renal function in cats post low-dose (111 MBq) radioiodine therapy.

Hyperthyroid cats presented for low-dose radioiodine therapy to The Feline Centre, Langford Vets were prospectively enrolled into the study. At baseline, one, six and 12 months post-radioiodine treatment, thyroid and renal function were evaluated via total thyroxine (TT4) and thyroid stimulating hormone (TSH), and via serum creatinine concentration (SCr) and GFR (corrected slope-intercept iohexol clearance), respectively. Cats were categorized as overt hypothyroid (TT4<15 nmol/l, TSH>0.15 ng/ml), subclinical hypothyroid (TT4≥ 15–60 nmol/l, TSH>0.15 ng/ml), euthyroid (TT4≤60 nmol/l, TSH≤0.15 ng/ml) and hyperthyroid (TT4>60 nmol/l) at each timepoint. Decreased GFR was defined as <0.92 ml/min/kg and azotaemia as SCr>175 µmol/l.

Twenty-seven cats were recruited into the study with 21 completing the 12-months follow-up; two were lost at one month and four at 12 months. Two cats (8%) remained persistently hyperthyroid, of which one underwent repeat radioiodine treatment. Five cats (20%) remained hyperthyroid at one-month but were euthyroid by 6 months. Seven cats (28%) developed overt hypothyroidism over the 12 months (2/7 at one-month, 4/7 at 6 months and 1/7 at 12 months) of which two were preceded by subclinical hypothyroidism and none of these cats regained normal thyroid function by 12 months. One cat had subclinical hypothyroidism at 12 months with no further follow-up available. No cats developed transient overt or subclinical hypothyroidism. Of the cats that developed overt hypothyroidism, 4/7 (57%) developed decreased renal function by 12 months. Decreased GFR preceded azotaemia development in 2/4 (50%) of these cats.

Twenty-eight percent of cats developed overt hypothyroidism that could be documented up to 12 months post treatment. This highlights the importance of continued monitoring of thyroid function post treatment even for cats receiving low-dose radioiodine therapy. Reduced renal function was documented in 57% of overtly hypothyroid cats, with GFR detecting decline in renal function earlier than SCr, suggesting GFR may be more useful to monitor renal function in cats post radioiodine therapy. However, the study is limited by the small number of cats developing overt hypothyroidism and further studies evaluating the optimal method for detecting early decline in renal function are required.

Disclosures

Disclosures to report:

This study was funded by a Petsavers grant and by Langford Vets clinical research fund NC Finch. Currently receives direct research support from Boehringer Animal Health, Petsavers, Langford Vets clinical research fund, Wellcome Trust. Currently receives indirect research support from Zoetis and IDEXX. Previously received direct support from Agria pet insurance. Previously received indirect support from MSD Animal Health. Salary is currently funded by the Wellcome Trust. Currently a member of the Governing Council of the Cat Fancy Veterinary Advisory Committee. Previously has received honorarium for speaking for not-for-profit organisations, and occasionally for pharmaceutical industries and for articles published in peer and non-peer reviewed publications (S. Tasker [ST]). In the past ST has received financial support for haemoplasma research from Zoetis Animal Health and for vector-borne disease research from Bayer Animal Health. ST receives financial support for current infectious disease research from BSAVA Petsavers, Langford Trust, Petplan Charitable Trust, Morris Animal Foundation, Dogs Trust, South West Biosciences DTP, MSD Animal Health and Langford Vets Clinical Research Fund. ST has also received financial support for infectious disease research in the past from the Elizabeth Blackwell Institute of the University of Bristol, ECVIM Clinical Studies Fund, the University of Bristol Campaigns and Alumni Fund, the RCVS Trust Fund Blue Skies and The Wellcome Trust. ST is a member of the World Forum for Companion Animal Vector Borne Diseases, supported by Bayer Animal Health. ST is a member of the European Advisory Board on Cat Diseases, which is supported by Merial. ST works for the Molecular Diagnostic Unit, Langford Vets, University of Bristol, which carried out the PCRs described in the study. ST has been paid for providing continuing professional development for not-for-profit organisations, and occasionally for commercial companies, around the world. A. Hibbert (AH) has received financial support for feline hyperthyroid disease research from BSAVA Petsavers, TOSOH Bioscience, Langford Vets and the Langford Trust. AH has been paid for providing continuing professional development for not-for-profit organisations, and occasionally for commercial companies, around the world.