Validation of an immunoassay for canine thyroid-stimulating hormone and changes in serum concentration following induction of hypothyroidism in dogs

Objective: To validate a new immunoradiometric assay for canine thyroid-stimulating hormone (cTSH) and to document changes in serum cTSH concentration during induction of hypothyroidism in dogs. Animals: Six healthy adult male Beagles. Procedure: Sensitivity, specificity, precision, and accuracy of the cTSH assay were evaluated in vitro. Hypothyroidism was induced in dogs by IV administration of sodium iodide I 131 solution. Subsequently, L-thyroxine was administered orally to normalize serum thyroxine concentrations. Results: The cTSH assay appeared to be specific and was sufficiently sensitive to detect cTSH in the serum of these dogs prior to induction of hypothyroidism. There was a 35-fold increase in mean serum cTSH concentration following induction of hypothyroidism, and 35 days after initiation of thyroid replacement therapy, mean serum cTSH concentration was not significantly greater than mean baseline value. Clinical Implications: Assay of serum cTSH is likely to prove helpful in the differential diagnosis of primary, secondary, and tertiary hypothyroidism in dogs, and in monitoring response to thyroid hormone replacement treatment.