Serum Cobalamin and Folate Concentrations in Cats with Hyperthyroidism.
SteinerJM, Peterson MA, Ruaux CG, Ryburn S, Williams DA.
Proceeding of the 23rd Annual Forum of the American College of Veterinary Internal Medicine, 2005.

Cats with hyperthyroidism commonly show vomiting and/or diarrhea, which are believed to be clinical signs due to direct effects of thyroid hormones on the gastrointestinal tract. However, signs of gastrointestinal disease do not resolve in some cats after 131I treatment. The goal of this study was to objectively assess intestinal function in cats with hyperthyroidism by measuring serum concentrations of cobalamin and folate.

Serum samples were collected from 13 cats with hyperthyroidism. The diagnosis of hyperthyroidism was based on clinical signs compatible with hyperthyroidism, a palpable thyroid nodule on physical examination, and an increased serum total T4 concentration (mean ±SD: 17.9 ±10.6 µg/dL) using a radioimmunoassay (Diagnostic Products Corporation, reference range: 0.8-4.0 µg/dL). Serum cobalamin and folate concentrations were measured by competitive binding chemiluminescence assays (Diagnostic Products Corporation).

Serum cobalamin concentrations were below the lower limit of the reference range (290 ng/L) in three of the 13 hyperthyroid cats (23.1%). Serum cobalamin concentrations were in the low end of the reference range in 4 additional cats (serum cobalamin concentrations: 302, 326, 426, and 448 ng/L). Serum cobalamin concentrations were well within the reference range in the remaining 6 cats. Serum folate concentrations were below the lower limit of the reference range (9.7 µg/L) in 5 of the 13 hyperthyroid cats (38.5%). Serum folate concentrations were in the low end of the reference range in an additional 3 cats (serum folate concentrations: 10.1, 10.5, and 10.8 µg/L). Serum folate concentrations were well within the reference range in the remaining 5 cats. A total of 6 of 13 cats (46.2%) had either a decreased serum cobalamin concentration, a decreased serum folate concentration, or both. Eleven of the 13 cats (84.6%) with hyperthyroidism had a decreased or low normal serum cobalamin concentration and/or a decreased or low normal serum folate concentration.

Serum cobalamin concentrations were either decreased or in the low end of the reference range in 53.8% of hyperthyroid cats evaluated in this study. Also, serum folate concentrations were either decreased or in the low end of the reference range in 61.5% of hyperthyroid cats evaluated in this study. These findings suggest the presence of significant malabsorption of both cobalamin and folate in cats with hyperthyroidism, although hypermetabolism may lead to increased vitamin requirements. Hyperthyroidism may directly or indirectly lead to vitamin malabsorption, or some hyperthyroid cats may have concurrent small intestinal disease. Whatever the mechanism, concurrent intestinal dysfunction and vitamin deficiencies appear to be common in hyperthyroid cats.