Serum Cobalamin and Methylmalonic Acid Concentrations in Hyperthyroid Cats Before and After Radioiodine Treatment

Background Hyperthyroidism, the most common endocrine disorder in cats, has been associated with low serum cobalamin concentrations. Whether this is a functional cobalamin deficiency of clinical importance has not been assessed. Hypothesis/Objectives Cats with hyperthyroidism experience a functional cobalamin deficiency which correlates with their clinical catabolic state and is reversible with return of the euthyroid state. Animals Thirty‐nine client‐owned hyperthyroid cats. Methods Prospective observational study. Serum cobalamin, methylmalonic acid, and clinical scores were determined in each hyperthyroid cat at enrollment and when euthyroid (60 days after radioiodine treatment). Results Five of the 39 hyperthyroid cats (13%) had a low serum cobalamin concentration ranging from <150 to 290 ng/L. Serum cobalamin concentrations normalized to >350 ng/L in 2 of the hypocobalaminemic cats once euthyroid. None of the hyperthyroid/hypocobalaminemic cats had increased serum methylmalonic acid concentrations (175–601 nmol/L). In cats with clinical and biochemical hyperthyroidism, there was no correlation between serum cobalamin concentrations with total T4 concentration ( P = .12) or clinical scores including body weight ( P = .11) and BCS ( P = .54). Conclusions and Clinical Importance In this population of hyperthyroid cats, the prevalence of hypocobalaminemia was low. Specifically, hyperthyroid cats, in which concurrent gastrointestinal disease is unlikely. Hypocobalaminemia is not a functional deficiency requiring supplementation in hyperthyroid cats without gastrointestinal disease.