Growth Hormone Deficiency: This is the most direct cause of low IGF-1 levels, as growth hormone (GH) stimulates the production of IGF-1 in the liver and other tissues. A deficiency in GH would logically lead to decreased production of IGF-1.

Malnutrition: Malnutrition can lead to low IGF-1 levels because adequate nutrition is necessary for the production of IGF-1. In states of malnutrition, the body may not have the necessary building blocks to produce IGF-1.
Chronic Diseases (e.g., Liver or Kidney Disease): Chronic diseases, especially those affecting the liver or kidneys, can impact the production or regulation of IGF-1. The liver is a primary site of IGF-1 production, and kidney disease can affect GH and IGF-1 regulation.
Hypothyroidism: Although less direct, hypothyroidism can affect growth hormone secretion and action, potentially leading to low IGF-1 levels.

Pituitary Tumors: While not the most common cause, pituitary tumors can lead to GH deficiency by either secreting inappropriate amounts of other hormones that suppress GH production or by physically compressing the normal pituitary tissue, leading to a deficiency in GH and subsequently low IGF-1.
Cranial Irradiation: History of cranial irradiation, especially in childhood, can lead to GH deficiency and subsequently low IGF-1 levels due to damage to the pituitary gland.
Genetic Disorders (e.g., Laron Syndrome): Although rare, genetic disorders like Laron Syndrome, which affects the GH receptor, can lead to low IGF-1 levels despite normal or elevated GH levels.

IGF-1 Gene Deletions or Mutations: Extremely rare conditions where there are deletions or mutations in the IGF-1 gene, leading to low or absent IGF-1 production.
African Pygmyism: A rare condition associated with low IGF-1 levels among other hormonal imbalances.
Other Rare Genetic Syndromes: Certain rare genetic syndromes can affect the GH/IGF-1 axis, leading to low IGF-1 levels, such as those affecting the GH receptor or post-receptor signaling pathways.