Clinical, routine laboratory and specific endocrine investigations in dogswith pituitary-dependent Cushing's syndrome: A comparison between dogs with macro- and dogs with microtumors.

It was investigated whether dogs with Gushing's syndrome with a very small pituitary tumor which cannot be identified by computed tomography (= microt umors) differ from dogs with visible tumors (= macrotumors) as related to s ignalement, clinical symptoms, results of routine laboratory tests and spec ific endocrine tests. The low-dose dexamethasone test was evaluated both as to a positive result as well as to a resistance or suppression respectivel y. Suppression was assumed if one or more of the following three criteria w ere present: a cortisol value < 1.4 <mu>g/dl 4 hours after administration o f dexamethasone, a cortisol value < 50 % of the basal value 4 hours after a dministration of dexamethasone, a cortisol value < 50 % of the basal value 8 hours after administration of dexamethasone. Ii the criteria were not met , a dexamethasone resistance was concluded. 22 of the 32 dogs (69 %) which were screened by computed tomography had a pituitary macrotumor. The dogs w ith macrotumors had a significantly higher body weight than the dogs with m icrotumors. As to age, sex, and results of the routine laboratory test and the ACTH stimulation test, there was no difference between the groups. The analysis of the low-dose dexamethasone test revealed that dogs with macrotu mors had far more frequently a dexamethasone resistance than the dogs with microtumors. In the group with macrotumors 84.6 % were dexamethasone resist ant whereas only 33.3 % in the group with microtumors. The results of this study allow the conclusion that the dexamethasone resistance of macrotumors proven by other investigators for the high-dose dexamethasone test holds a lso true for the low-dose dexamethasone test. Additionally, large breed dog s seem at higher risk for the development of large pituitary tumors.