Effects of Toceranib Phosphate on the Hypothalamic‐Pituitary‐Thyroid Axis in Tumor‐Bearing Dogs

Background Thyroid dysfunction is associated with the use of tyrosine kinase inhibitors (TKI) in people. Hypothesis/Objectives To determine whether dysfunction in the hypothalamic‐pituitary‐thyroid axis occurs in dogs receiving the TKI, toceranib phosphate. Animals Forty‐three client‐owned dogs with cancer. Methods Prospective, observational study. Concentrations of total thyroxine (TT4), free thyroxine (FT4), total triiodothyronine (TT3), and thyroid‐stimulating hormone (TSH) were evaluated on day 0, 30, and 90. Dogs also were evaluated for the presence of thyroglobulin autoantibodies. Results The proportion of dogs with low TT4, low FT4, low TT3, high TSH, or primary hypothyroidism (increased TSH and decreased TT4, FT4 or both) did not change over 90 days. Hormone concentrations remained within laboratory reference intervals, but FT4 (P = 0.0032) and TSH (P < 0.0001) changed over time. Mean FT4 was 1.22 ng/dL (95% confidence interval [CI], 1.10–1.34) on day 0 and 1.00 ng/dL (95% CI, 0.86–1.16) on day 90. Mean TSH was 0.17 ng/mL (95% CI, 0.13–0.23) on day 0 and 0.34 ng/mL (95% CI, 0.24–0.48) on day 90. Furthermore, TT4/TT3 ratio also changed over time (P = 0.0086). Mean TT4/TT3 ratio was 2.57 (95% CI, 2.26–2.88) on day 0 and 2.02 on day 90 (95% CI, 1.61–2.44). Thyroglobulin autoantibodies were not detected in any dog. Conclusions and Clinical Importance Toceranib phosphate can disrupt the hypothalamic‐pituitary‐thyroid axis in dogs. Periodic evaluation of TT4, FT4, TT3, and TSH should be carried out in dogs receiving long‐term treatment with this medication.