High urinary corticoid/creatinine ratios in cats with hyperthyroidism

Measurement of the urinary corticoid : creatinine (C : C) ratio provides an assessment of cortisol secretion over a period of time. Therefore, this test is a very sensitive measure of adrenocortical function. The stress of the diagnostic procedure and nonadrenal disease may increase the urinary C : C ratio. In addition, diseases such as hyperthyroidism may influence the metabolic clearance of cortisol. To evaluate the effect of thyroid hormone excess, urinary C : C ratios were measured in 32 cats with hyperthyroidism and 45 healthy household cats. In 7 cats, urinary C : C ratios were measured both before and after treatment for hyperthyroidism. With data from the healthy cats, the reference range for the urinary C : C ratio was determined to be 8.0 to 42.0 X 10(-6). The urinary C : C ratios in the cats with hyperthyroidism (median, 37.5 x 10(-6); range, 5.9-169.5 x 10(-6)) were significantly (P = .001) higher than those in the healthy cats (median, 16 x 10(-6); range, 4.8-52.5 x 10(-6)). In 15 cats with hyperthyroidism, the urinary C : C ratios exceeded the upper limit of the reference range. Treatment for hyperthyroidism led to a marked decrease in urinary C : C ratios. The results of this study demonstrate that the urinary C : C ratio may be abnormally high in cats with hyperthyroidism, probably because of increased metabolic clearance of cortisol and activation of the pituitary-adrenocortical axis by disease. Although the clinical features of hyperthyroidism and hyperadrenocorticism in cats are different, hyperthyroidism should be ruled out when cats are suspected of hyperadrenocorticism on the basis of abnormally high urinary C : C ratios.