IGF1 and Insulin resistance: Furthermore, although a positive predictive value of 95% for IGF-1 > 1000 ng/mL measured by RIA was reported in a large study including 1221 diabetic cats (319 had IGF-1 > 1000 ng/mL, 63/319 had cranial imaging or necropsy, and HS was confirmed in 60/63) [1], ideally, results of cranial imaging and clinical suspicion (e.g., typical phenotypic features and evidence of insulin resistance) should be considered when making a diagnosis of HS.