Ittoop T, Reddy SSK. Insulin-like Growth Factor-1 Deficiency. In: McDermott MT, ed. Management of Patients with Pseudo-Endocrine Disorders: A Case-Based Pocket Guide. Cham: Springer International Publishing; 2019:235-49. Insulin-like Growth Factor-1 Deficiency Since insulin-like growth factor-1 (igf-1) measurement has become widely known to reflect cumulative growth hormone (GH) secretion, it is not surprising for clinicians to be tempted to use IGF-1 levels to not only screen for GH excess but for GH deficiency also (Le Roith D. Seminars in medicine of the Beth Israel Deaconess Medical Center. Insulin-like growth factors. N Engl J Med. 1997;336:633–40). Can low IGF-1 levels be used as a surrogate indicator of reduced GH secretion? Are low IGF-1 levels prognostic of other pathologies? To interpret IGF-1 levels properly, it is important to understand IGF-1/GH regulation and related variables. The causes of IGF-1 deficiency can be classified into two main types: primary and secondary. Primary IGF-1 deficiency refers to IGF-1 impairment but with normal GH secretion. Secondary IGF-1 deficiency refers to IGF-1 impairment due to some dysregulation in GH secretion. Low IGF-1 levels without no GH related issues, may be a prognostic indicator of one of the individual’s co-morbidities.