The prevalence of hypocobalaminaemia in cats with spontaneous hyperthyroidism.

Objectives: To determine the prevalence of hypocobalaminaemia in cats with moderate to severe hyperthyroidism and to investigate the relationship between cobalamin status and selected haematologic parameters. Methods: Serum cobalamin concentrations were measured in 76 spontaneously hyperthyroid cats [serum thyroxine (T4) concentration ≥100 nmol/L] and 100 geriatric euthyroid cats. Erythrocyte and neutrophil counts in hyperthyroid cats with hypocobalaminaemia were compared with those in hyperthyroid cats with adequate serum cobalamin concentrations (≥290 ng/L). Results: The median cobalamin concentration in hyperthyroid cats was lower than the control group (409 versus 672 ng/L; P=0·0040). In addition, 40·8% of hyperthyroid cats had subnormal serum cobalamin concentrations compared with 25% of controls (P=0·0336). Weak negative correlation (coefficient: –0·3281) was demonstrated between serum cobalamin and T4 concentrations in the hyperthyroid population, and the median cobalamin concentration was lower in cats with T4 above the median of 153 nmol/L compared with cats with T4 below this value (P=0·0281). Hypocobalaminaemia was not associated with neutropenia or anaemia in hyperthyroid cats. Clinical Significance: This study indicates that a substantial proportion of cats with T4≥100 nmol/L are hypocobalaminaemic and suggests that hyperthyroidism directly or indirectly affects cobalamin uptake, excretion or utilisation in this species.